Your bike shop is sorta right. Conversion can be a can of worms if you're not careful. Also, have you considered the geometry issue? Frames designed for drop bars tend towards shorter top tubes and sometimes taller head tubes.
I'll share my experience though, because I've been around this block once. A few years ago I built a Salsa Fargo with flat bars only to later convert it to drops. Brifters -- integrated shifters and brakes -- were too expensive and would have required me to also replace my mountain brake calipers with road brake calipers. Instead, I went with some inexpensive Cane Creek (Tektro also sells them) brake levers for drop bars that pulled cable in the mountain-bike ratio. Then I tossed on some bar-end shifters. I also had to replace all the shifter and brake cables and some housing because of the extra length needed to reach the drop bar and go under the bar tape.
It was an expensive conversion.
The products Nightshift links to are interesting. I have never tried clamp-on drops for mountain-bars. I probably won't ever try them either, but at least there is a less-expensive option to experiment with.
FWIW, I also like the idea of looking at alternate styles of flat bars. There are a surprising number of designs out there.