Fix undercarbonated bottles

I have a keg setup, but wanted to bottle condition a strong Belgian ale. Anyway, I think I didn't mix in the priming sugar well because I have about half of the bottles that are severely undercarbonated. They have been bottled for about 4 weeks. Any ideas on how to carbonate these without ruining the beer? Two thoughts I have are to pour bottles into a keg and force carbonate (big oxidation risk) or add more sugar to each bottle. Anyone try this before?

If you have random bottles that are undercarbonated, buy one or make a few carbonator caps. When you find a bottle that is flat, dump it in a plastic bottle, squeeze the air out, charge it up with CO2, give it a shake, enjoy.