The WCP DS is not designed to mount an encoder, as for shifting gearboxes this will cause complications when writing code to calculate distance travelled etc. However, there is a very nice 3D printed part which should allow you to do this. It utilises the CUI encoders that were available in FIRST choice this year.
http://www.chiefdelphi.com/forums/sh...13#post1502113
For a WCD the most common thing, is either drilling a hole in an axle shaft and using a grayhill encoder. But it can have alignment related issues. The other method, is turning the shaft down on the lathe to a rod, the same diameter as the encoder shaft, and joining them with a flexible shaft coupling or just a piece of KOP surgical tube( backlash is not a major problem).
You could also try to use the new CTR mag encoders.
http://www.ctr-electronics.com/senso...c-encoder.html