I would like to see if I could use some kind of touch switch or sensor instead of the mechanical micro switch.
That should be quite easy, as it seems to be a simple switch function (i.e. closed when pressed and open when released).
The best way to do this is probably to connect the sensor to a microcontroller, and then attach the microcontroller to the wheelchair input system with a cord that has a 3.5 mm plug. How would I attach something like that to my icrocontroller?
If you don't have the microcontroller allready, then there is no need to include one.
A touch sensor is easy to make (remember, if it's for a disabled person, it should be properly build and safe) and there's a good chance that an open collector transistor on the output can replace the actual switch contacts (if orientated the right way round).
Tell us what exactly it's going to control and it will be easier to help.
For a quick test to see if a transistor will control the input in question, you can try bridging the contacts (the poles in a similar 3.5mm connector) with a diode. If it doesn't work first time, try turning the diode around.