I don't think that can be done. There is no place for the derailleur to hang and the wheel will be bigger to accommodate the gears. New wheel $80, derailleur $50, shifter $80, and $20 chain.
I think the easiest way would be to get an internal hub with 7 or 8 gears and have your wheel remade. This could be very expensive may be easier to get a new bike. The Hub will cost $200, $50+spokes to build a wheel, then you need a shifter $70.