Rather than bloat Cubasis with customizable keyboards I use what is already there.
Just verified that it works. You can control Cubasis instruments, including IAA, with virtual keyboards from other apps. I tested with Sampletank loaded as an IAA on a Cubasis midi track and Musix Pro which allows an almost infinite configuration of layouts, scales and modes. Works fine. Also tested with Cubasis built in instruments. Works.
So I assume any Midi out app can be used. If you like the Magellan keyboard or Sound Prism or Geo or ThumbJam, use them.