diff --git a/pungi/checks.py b/pungi/checks.py
index 7eb98a2a..c67c88b9 100644
--- a/pungi/checks.py
+++ b/pungi/checks.py
@@ -185,12 +185,12 @@ def _check_dep(name, value, lst, matcher, fmt):
             yield fmt.format(name, value, dep)
 
 
-def validate(config, offline=False):
+def validate(config, offline=False, schema=None):
     """Test the configuration against schema.
 
     Undefined values for which a default value exists will be filled in.
     """
-    schema = make_schema()
+    schema = schema or make_schema()
     DefaultValidator = _extend_with_default_and_alias(
         jsonschema.Draft4Validator, offline=offline
     )
@@ -1442,6 +1442,25 @@ CONFIG_DEPS = {
 }
 
 
+def update_schema(schema, update_dict):
+    """
+    Updates the leaf values in `schema` by the leaf values from `update_dict`.
+
+    This is different from `schema.update(update_dict)`, because
+    the `schema.update()` would override also non-leaf values.
+
+    :param schema: Schema to update.
+    :param updated_dict: Dict matching the schema with updated leaf values.
+    :returns: Updated schema
+    """
+    for k, v in update_dict.items():
+        if isinstance(v, dict):
+            schema[k] = update_schema(schema.get(k, {}), v)
+        else:
+            schema[k] = v
+    return schema
+
+
 def _get_gather_backends():
     if six.PY2:
         return ['yum', 'dnf']
diff --git a/pungi/scripts/config_validate.py b/pungi/scripts/config_validate.py
index 0fa57689..c73e3b91 100644
--- a/pungi/scripts/config_validate.py
+++ b/pungi/scripts/config_validate.py
@@ -72,7 +72,7 @@ def read_variants(compose, config):
             compose.all_variants[child.uid] = child
 
 
-def run(config, topdir, has_old, offline, defined_variables):
+def run(config, topdir, has_old, offline, defined_variables, schema_overrides):
     # Load default values for undefined variables. This is useful for
     # validating templates that are supposed to be filled in later with
     # pungi-config-dump.
@@ -89,7 +89,13 @@ def run(config, topdir, has_old, offline, defined_variables):
     # Remove the dummy variables used for defaults.
     config_utils.remove_unknown(conf, defined_variables)
 
-    errors, warnings = pungi.checks.validate(conf, offline=offline)
+    # Load extra schemas JSON files.
+    schema = pungi.checks.make_schema()
+    for schema_override in schema_overrides:
+        with open(schema_override) as f:
+            schema = pungi.checks.update_schema(schema, json.load(f))
+
+    errors, warnings = pungi.checks.validate(conf, offline=offline, schema=schema)
     if errors or warnings:
         for error in errors + warnings:
             print(error)
@@ -169,11 +175,21 @@ def main(args=None):
             "Can be used multiple times."
         ),
     )
+    parser.add_argument(
+        '--schema-override',
+        action="append",
+        default=[],
+        help=(
+            'Path to extra JSON schema defining the values which will override '
+            'the original Pungi JSON schema values.'
+        ),
+    )
     opts = parser.parse_args(args)
     defines = config_utils.extract_defines(opts.define)
 
     with pungi.util.temp_dir() as topdir:
-        errors = run(opts.config, topdir, opts.old_composes, opts.offline, defines)
+        errors = run(opts.config, topdir, opts.old_composes, opts.offline, defines,
+                     opts.schema_override)
 
     for msg in errors:
         print(msg)
diff --git a/tests/data/dummy-override.json b/tests/data/dummy-override.json
new file mode 100644
index 00000000..39189f62
--- /dev/null
+++ b/tests/data/dummy-override.json
@@ -0,0 +1,7 @@
+{
+    "properties": {
+        "pkgset_source": {
+            "enum": ["koji"]
+        }
+    }
+}
diff --git a/tests/test_checks.py b/tests/test_checks.py
index 299bca80..4b945a0d 100644
--- a/tests/test_checks.py
+++ b/tests/test_checks.py
@@ -542,6 +542,31 @@ class TestSchemaValidator(unittest.TestCase):
         self.assertEqual(config["foo"], "git://example.com/repo.git#HEAD")
         self.assertEqual(resolve_git_url.call_args_list, [])
 
+    def test_update_schema(self):
+        schema = checks.make_schema()
+        schema_override = {
+            "definitions": {
+                "scm_dict": {
+                    "properties": {
+                        "scm": {
+                            "enum": ["git"]
+                        },
+                        "repo": {
+                            "enum": ["git://localhost/pungi-fedora.git"]
+                        },
+                    }
+                }
+            }
+        }
+        schema = checks.update_schema(schema, schema_override)
+        scm_dict_properties = schema["definitions"]["scm_dict"]["properties"]
+        self.assertEqual(
+            scm_dict_properties["scm"],
+            {'enum': ['git'], 'type': 'string'})
+        self.assertEqual(
+            scm_dict_properties["repo"],
+            {'enum': ['git://localhost/pungi-fedora.git'], 'type': 'string'})
+        self.assertEqual(scm_dict_properties["file"], {"type": "string"})
 
 class TestUmask(unittest.TestCase):
     def setUp(self):
diff --git a/tests/test_config_validate_script.py b/tests/test_config_validate_script.py
index 961e8701..542f35b3 100644
--- a/tests/test_config_validate_script.py
+++ b/tests/test_config_validate_script.py
@@ -13,6 +13,7 @@ from tests import helpers
 
 HERE = os.path.abspath(os.path.dirname(__file__))
 DUMMY_CONFIG = os.path.join(HERE, 'data/dummy-pungi.conf')
+SCHEMA_OVERRIDE = os.path.join(HERE, 'data/dummy-override.json')
 
 
 class ConfigValidateScriptTest(helpers.PungiTestCase):
@@ -24,3 +25,16 @@ class ConfigValidateScriptTest(helpers.PungiTestCase):
         cli_main()
         self.assertEqual('', stdout.getvalue())
         self.assertEqual('', stderr.getvalue())
+
+    @mock.patch('sys.argv', new=[
+        'pungi-config-validate', DUMMY_CONFIG, "--schema-override",
+        SCHEMA_OVERRIDE])
+    @mock.patch('sys.stderr', new_callable=six.StringIO)
+    @mock.patch('sys.stdout', new_callable=six.StringIO)
+    @mock.patch('sys.exit')
+    def test_schema_override(self, exit, stdout, stderr):
+        cli_main()
+        self.assertTrue(stdout.getvalue().startswith(
+            "Failed validation in pkgset_source: 'repos' is not one of"))
+        self.assertEqual('', stderr.getvalue())
+        exit.assert_called_once_with(1)