My first idea is I would look into the hex you have for the rims to see if it would be possible to drill and tap a set screw in to the hex. I would use metal hex's depending on how much torque your gear motors are. Low torque use a set screw. High torque make a key way and machine the motor shaft to the hex.
Here is an example but these are 12mm hex.
http://store.rc4wd.com/Thin-12mm-Whe...le_p_2255.html
Clamping hex's are also an option but I wouldn't use this type. As it is only to keep it on so you don't loose your axle pins. Unless you use them.
http://www.etamiya.com/shop/tamiya-5...er-p-4642.html
One other option and probably the way I would go is just to take your stuff you have and drill a hole in the gear motor shaft to slip in an axle pin just like it would be on a regular axle.
Either way you figure it out. Once your hex is mounted. How to mount the actually wheel so it bolts on? Well do your gear motors have a slot already for a set screw? If you can find them without it then just thread the end. Modifying the shaft might be what you have to do. Machine it down to a usable diameter to use a die to thread the shaft.
What size wheels are you looking to put on the 14 and 17mm hex's? Whats the size and power of your gear motor? Output shaft diameter? Rpm?
There must be someone else on here who has actually done this I'm sure. Maybe use a set screw as part of the rim itself??
Maybe forget the hex's. Go this way. Drill holes in your rim to an adapter like this.
http://www.servocity.com/html/wheel_..._wad5130_.html
Or use something like this. 12mm hex. 12mm to 14mm hex can be added.
http://www.lynxmotion.com/p-591-12mm...haft-pair.aspx
I have yet to find anything directly to 14 or 17mm.