I'd probably just get Cane Creek Crosstop levers (or similar). Install on the flat portions of the bar and work in addition to current levers. You should be able to get 'em installed for $40-70 if you've a good bike shop around. If you install 'em yourself, they're as low as $22 on the internet.

To do a flat bar conversion, you need to determine the clamp diameter of your stem. If it's a 26mm, you'll need new stem, bar, shift levers and brake levers.

Figure for the conversion:
$15 stem
$15 bar
$15 for falcon friction shifters, if you can live without the clicky. (might be able to score 7-sp index off ebay for about the same price)
$17 brake levers
$10 grips
$10 cables/housing

$82 + shipping.
Add money for tools like a cable cutter.

Oh, and do 200 twisting crunches and at least 20 pushups a day. Pretty soon your back should stop hurting.