The big problem with converting manual mills is mounting the ballscrew nuts.
A ballscrew nut typically takes up a lot more room than a leadscrew nut, so mounting new nuts can be a challenge.

My advise, is if you are serious about it, is strip it, do lots of measuring, and see if you can physically fit ballscrews/nuts in. If you can, reassemble it, machine the bits you need, then strip and rebuild it again with the new parts.
Other than the weight of bits, especially the table, which you can often get away with just sliding along far enough to get access without completely removing it, mills aren't that hard to dismantle.