Is it possible to submit new GRE scores after having applied to an acadmic program?
Honestly, the GRE scores are perhaps the most useless part of the application record. Good scores tell me that someone speaks English well, and is capable of basic mathematics. A bad score could just be a signpost of having a bad day, or perhaps some issues with English that don't necessarily translate into being a bad student or a bad researcher. (Just about the only thing GRE scores tend to correlate with in grad school is the performance in first-year classes!)

If you do have replacement scores, you can definitely send them to grad schools. They'll be considered, but they're not likely to sway a decision (unless the school has a minimum cutoff—which is possible for international students).