You can, but you don't need to add users or groups in DB2. They automatically get added to the DB2 system catalog (TABAUTH, DBAUTH, etc) when you issue a SQL GRANT to a user or group. The actual password authentication is handled by the OS (so the account must exist in the OS), but INSERT, UPDATE, SELECT, CONNECT, etc authorities within DB2 are handled with SQL GRANT, REVOKE, etc.
GRANTS to user or group accounts can be made within DB2 before the accounts are created in the OS. They only get checked when authentication takes place.