Actually it is possible, take a look at this FAQ, basically you need a cartridge that plugs into the megadrive/genesis and this allows foreign titles to be played.
http://www.angelfire.com/ny/dezmoowu/Sega/bcantin.txt
Another way of doing it is to make copies of your ntsc games, then there is a program which can change the region on the CD's.
I can't rememeber it's name at the moment but if you can't find any infomation on it I'll see if I can find it
edit
found it! http://www.pages.drexel.edu/~mrp29/convscd102.zip