I have CNC 'd a 7x12. If you use TurboCNC, you can create two different .INI files and call them Mill and Lathe, and then just load whichever one you want to use at the time.
What are you planning to use your machine for? The 7x lathe is never going to be a very satisfactory mill bed. By the time you go to all the effort to convert a cheap drill into a mill head, you would be better off putting that effort into an evening job at the gas station for a couple of months then just buy a mini mill. It will take less time and give better results.