You could probably rig up the drill with some large hose clamps. Then take a smaller one to clamp down the trigger to whatever speed you want, then wire it into a foot pedal from a sewing machine.

If you don't want to wire it in, they make pedals that directly plug in, like this.

http://www.ebay.com/itm/Foot-Pedal-C...#ht_1356wt_808

I wouldn't pay $30 though.