I would think so, You would most likely need to relace the 3 speed hub with beefier spokes and to a stronger rim. You would also need to run the cables for the shifter that may take a bit of imagination but It could most likely be done. I'm no expert just an opinion - CJ
You gotta find a better bike shop. That job should be easy. Just buy a three speed hub and shifter and lace it to your wheel. Might need to buy extra long cable, but other than that, it should work same as a regular bike.