I'm sure it's possible, you'd just need to build a 16/20/24 inch wheel (whichever it is) around a flip flop hub. Chainline issues can be resolved fairly easily.

the real quiestion is why?

then again, if you're just looking for something entertaining to do, and your competent building a wheel, or just want to learn, there shouldn't be any real problem with this. I give it my blessing, assuming you have time and money to spend.