Add new field with auto-icrement to existing table

Dear all,

I have a database structure with several default empty tables defines in an
admin schema. For example :

CREATE TABLE admin."ConfigurableWindowGUIEntryInternal"
(
"DbSyncID" bigint NOT NULL,
"DbSyncInsertedBackupVersion" bigint NOT NULL,
...
)
WITH (
OIDS=FALSE
);

And, for each client, I have a dedicated schema with tables that derive
from those admin tables, and use the two first elements as primary keys
(DbSyncID and DbSyncInsertedBackupVersion). For example :

CREATE TABLE user_1."ConfigurableWindowGUIEntryInternal"
(
CONSTRAINT "ConfigurableWindowGUIEntry_pk" PRIMARY KEY ("DbSyncID",
"DbSyncInsertedBackupVersion")
)
INHERITS (admin."ConfigurableWindowGUIEntryInternal")
WITH (
OIDS=FALSE
);

This schema is used for synchronisation of the database between the server
and client applications and I do not want to change anything in this
communication protocol.

But now, I would want to develop a server-based application. And this
application would need a single column unique ID for referencing tables
entries. My current primary key does not fit my needs as it is based on two
columns. So, I would want to add a new field on all derived tables. And, as
I don't want to change the synchronisation stuff, the values of this new
field should be assigned automatically.

Is there a way to add this once for all by changing the parent table or do
I need to add the field for each derived table ? And how can I perform
this, taking into account that the tables already contain data ?

Thanks in advance for your help.

Regards,
Brice