DISCLAIMER: I have not used the generic client to access either Oracle or DB2, so I'm speaking in hypothetical here.
.NetTiers uses a provider model in the data tier. There are two flavors of providers that come out-of-the-box. SQL Server and "Generic". You can perform your Oracle and DB2 using the generic providers but will not have tight integration with any DBMS-specific features.
You could also take a copy of the templates for an existing provider and tweak them to suit your actual database. This would probably be trivial. Particularly for Oracle when you'll have a native support from .NET (if you use DB2, you might choose to use a third-party .NET provider like DataDirect).
-C