The biggest thing about reheating in a roaster is that the heating element is on the bottom, So you have to keep the temp fairly low and /or use a rack or pan to raise them off the very bottom. Also, if they are frozen and in a thick layer, the edges get dry and overcooked before the middle of the layer gets done.

So, check out the frozen ones at the store and notice the way they are packaged and the heating instructions. If you keep the layer the same as the packaged ones or no more than 2 deep,keep the temp on the low side (200-300), maybe even put a layer of sauce in the bottom, you should be good- like a giant slow cooker. If you have a lot, you may want 2 roasxters to keep the layers reasonable.