For this application you are really looking for a shaft adapter to go from the CIM motor output shaft to the correct diameter for your wheel. As a very basic prototyping tool we installed a CIM Motor to a Banebots Planetary gearbox with the appropriate gears removed to give an output shaft diameter of .5" and gear ratio of 1:1. However, this can be somewhat complicated to do and we had it laying around in our work area so it was an easy prototyping tool. I would suggest you go with something like a Helical Beam Shaft Coupler from Mcmaster-Carr,
http://www.mcmaster.com/#standard-sh...plings/=fw06xw
But that is just a suggestion and I'm sure some other people will have better options and/or suggestions.