You have two options as far as I know (without having to install extra software that is).
1. A lot of instruments have routing settings that allow you to control parameters using Aftertouch
2. A lot of keyboards allow you to change the CC# of your aftertouch to something that is more basic. After changing your pressure control to a simple CC#, you should be able to assign it much more easily.
The fun part is trying to calibrate your min/max to something that isn't overkill

Aftertouch can get quite jumpy sometimes.