Sure can just pop in a coopers carb tablet in the beer, or measure out the appropriate amount of dextrose or table sugar. Cap the beer and wait your 3 weeks.
It'll work, but be really careful. I added carb tabs to some beers that I thought weren't going to carb, but either they were slow carbers or weren't warm enough, because I got a couple bottle bombs. So before you add more sugar, make damn sure you're not overdoing it.