Cascade midi controller ?

Actually my silent system (Quietime Magic Star) is connected via a MIDI interface (MIDI OUT to MIDI IN) to my iMac and I can play with Pianoteq instruments.

I don't have any other keys except my keybord piano which I use to play, I see that is possible to assign MIDI events to pianoteq in order to do various things like select specific preset, manage volume, maybe metronome ?, etc.

Should be possible to link a new (midi controller ?) to my setup to be able doing above ?
My system has also a MIDI THROUGH port...

Suggestions ? I wouldn't pay a lot of money.