.. how can I assign other midi controller values to
different switches and knobs of my keyboard?
this is something you do with the keyboard rather than with GarageBand, so you'll have to research it from that aspect. GarageBand is very limited in what it will accept. If you use SoundFontSynth to access the QuickTime Musical Instruments you can send the MIDI numbers and change instruments from the keyboard. You can assign different functions to the modulation wheel by sending CC messages: 01=modulation (default), 07=channel volume, 10=pan, 128=Pitch Bend sensitivity (this last enables you to increase the amount of pitch bend, but not with GarageBand instruments, only with SoundFonts and QT instruments).
Stay away from CC 120 to 127 which could screw things up.
Note that any settings you change, such as pitch bend sensitivity, will be reset on restarting GarageBand.