The problem with trying to increase offset of a wheel once its already out of the manufacturing process is getting the actual wheel square to the machine. If you are at all, even a few thousandths of an inch, out of square to the machine, you will get some wheel wobble and probably a difficult time balancing the wheel, as well as vibration when rolling.
I would stay away from trying to get wheels with to low an offset to fit by machining the face that mates with the hub. Just find wheels that are designed to fit.
Also, buying wheels off ebay in the incorrect offset for <$900 a set NEVER ends well.