You can open the bottles, add sugar, and recap. Not simple, but it will work. The little carbonation drops are probably the easiest way to do this. Try it with a few bottles, wait 10ish days and see if you like it. Carbonation may not be perfect, but it will be better.
I put beers that I don't want to drink (overcarbed, old, etc) in a case near the kitchen and use them for cooking.