diff --git a/swh/core/api/serializers.py b/swh/core/api/serializers.py
--- a/swh/core/api/serializers.py
+++ b/swh/core/api/serializers.py
@@ -42,21 +42,23 @@
 }
 
 
-def encode_data_client(data: Any) -> bytes:
+def encode_data_client(data: Any, extra_encoders=None) -> bytes:
     try:
-        return msgpack_dumps(data)
+        return msgpack_dumps(data, extra_encoders=extra_encoders)
     except OverflowError as e:
         raise ValueError('Limits were reached. Please, check your input.\n' +
                          str(e))
 
 
-def decode_response(response: Response) -> Any:
+def decode_response(response: Response, extra_decoders=None) -> Any:
     content_type = response.headers['content-type']
 
     if content_type.startswith('application/x-msgpack'):
-        r = msgpack_loads(response.content)
+        r = msgpack_loads(response.content,
+                          extra_decoders=extra_decoders)
     elif content_type.startswith('application/json'):
-        r = json.loads(response.text, cls=SWHJSONDecoder)
+        r = json.loads(response.text, cls=SWHJSONDecoder,
+                       extra_decoders=extra_decoders)
     elif content_type.startswith('text/'):
         r = response.text
     else:
@@ -90,9 +92,15 @@
 
     """
 
+    def __init__(self, extra_encoders=None, **kwargs):
+        super().__init__(**kwargs)
+        self.encoders = ENCODERS
+        if extra_encoders:
+            self.encoders += extra_encoders
+
     def default(self, o: Any
                 ) -> Union[Dict[str, Union[Dict[str, int], str]], list]:
-        for (type_, type_name, encoder) in ENCODERS:
+        for (type_, type_name, encoder) in self.encoders:
             if isinstance(o, type_):
                 return {
                     'swhtype': type_name,
@@ -129,12 +137,18 @@
 
     """
 
+    def __init__(self, extra_decoders=None, **kwargs):
+        super().__init__(**kwargs)
+        self.decoders = DECODERS
+        if extra_decoders:
+            self.decoders = {**self.decoders, **extra_decoders}
+
     def decode_data(self, o: Any) -> Any:
         if isinstance(o, dict):
             if set(o.keys()) == {'d', 'swhtype'}:
                 if o['swhtype'] == 'bytes':
                     return base64.b85decode(o['d'])
-                decoder = DECODERS.get(o['swhtype'])
+                decoder = self.decoders.get(o['swhtype'])
                 if decoder:
                     return decoder(self.decode_data(o['d']))
             return {key: self.decode_data(value) for key, value in o.items()}
@@ -148,13 +162,17 @@
         return self.decode_data(data), index
 
 
-def msgpack_dumps(data: Any) -> bytes:
+def msgpack_dumps(data: Any, extra_encoders=None) -> bytes:
     """Write data as a msgpack stream"""
+    encoders = ENCODERS
+    if extra_encoders:
+        encoders += extra_encoders
+
     def encode_types(obj):
         if isinstance(obj, types.GeneratorType):
             return list(obj)
 
-        for (type_, type_name, encoder) in ENCODERS:
+        for (type_, type_name, encoder) in encoders:
             if isinstance(obj, type_):
                 return {
                     b'swhtype': type_name,
@@ -165,11 +183,15 @@
     return msgpack.packb(data, use_bin_type=True, default=encode_types)
 
 
-def msgpack_loads(data: bytes) -> Any:
+def msgpack_loads(data: bytes, extra_decoders=None) -> Any:
     """Read data as a msgpack stream"""
+    decoders = DECODERS
+    if extra_decoders:
+        decoders = {**decoders, **extra_decoders}
+
     def decode_types(obj):
         if set(obj.keys()) == {b'd', b'swhtype'}:
-            decoder = DECODERS.get(obj[b'swhtype'])
+            decoder = decoders.get(obj[b'swhtype'])
             if decoder:
                 return decoder(obj[b'd'])
         return obj
diff --git a/swh/core/api/tests/test_serializers.py b/swh/core/api/tests/test_serializers.py
--- a/swh/core/api/tests/test_serializers.py
+++ b/swh/core/api/tests/test_serializers.py
@@ -21,6 +21,28 @@
 )
 
 
+class ExtraType:
+    def __init__(self, arg1, arg2):
+        self.arg1 = arg1
+        self.arg2 = arg2
+
+    def __repr__(self):
+        return f'ExtraType({self.arg1}, {self.arg2})'
+
+    def __eq__(self, other):
+        return (self.arg1, self.arg2) == (other.arg1, other.arg2)
+
+
+extra_encoders = [
+    (ExtraType, 'extratype', lambda o: (o.arg1, o.arg2))
+]
+
+
+extra_decoders = {
+    'extratype': lambda o: ExtraType(*o),
+}
+
+
 class Serializers(unittest.TestCase):
     def setUp(self):
         self.tz = datetime.timezone(datetime.timedelta(minutes=118))
@@ -67,6 +89,16 @@
         data = json.dumps(self.data, cls=SWHJSONEncoder)
         self.assertEqual(self.data, json.loads(data, cls=SWHJSONDecoder))
 
+    def test_round_trip_json_extra_types(self):
+        original_data = [ExtraType('baz', self.data), 'qux']
+
+        data = json.dumps(original_data, cls=SWHJSONEncoder,
+                          extra_encoders=extra_encoders)
+        self.assertEqual(
+            original_data,
+            json.loads(
+                data, cls=SWHJSONDecoder, extra_decoders=extra_decoders))
+
     def test_encode_swh_json(self):
         data = json.dumps(self.data, cls=SWHJSONEncoder)
         self.assertEqual(self.encoded_data, json.loads(data))
@@ -75,6 +107,13 @@
         data = msgpack_dumps(self.data)
         self.assertEqual(self.data, msgpack_loads(data))
 
+    def test_round_trip_msgpack_extra_types(self):
+        original_data = [ExtraType('baz', self.data), 'qux']
+
+        data = msgpack_dumps(original_data, extra_encoders=extra_encoders)
+        self.assertEqual(
+            original_data, msgpack_loads(data, extra_decoders=extra_decoders))
+
     def test_generator_json(self):
         data = json.dumps(self.generator, cls=SWHJSONEncoder)
         self.assertEqual(self.gen_lst, json.loads(data, cls=SWHJSONDecoder))