Can a colt AR 15 PreBan legally be made full auto?

It is possible, even for a postban receiver, under a specific set of circumstances. You would need to obtain an NFA Class III tax stamp to purchase a registered trigger group, sear, or lightning link, and the person actually doing the conversion would have to hold a Type 7 FFL with a Class III Special Occupational Tax stamp (as well as a CNC milling machine, as the parts do not simply drop in).