Hmmm ... I did exactly this, i.e. placed the jar file in the lib folder and also restarted RA.
If I then goto "Datenbankverbindungen" --> "Neue Verbindung einrichten" (sorry I've only the german translation)
I get all databases under "Datenbanksystem" that where delivred originally but not the new one (what's not surprising,
since even no classpath adaptation was done, etc.)
So I guess that there is to do a little bit more than just copying the jar file.

By the way: I want to use IBM Informix.