Umm you should be able to add to them, I don't see why not. As long as you chill the new EBM before adding to whats already in the bag. Can't see why it would be any different to adding to a bottle KWIM?
However you can't reuse the bags so if your looking for something that you can reuse look out for cheap baby bottles to use. You can usually find the little ones pretty cheap in the junk shops.

The bags are single use, well at least the last time I looked at them they were but I haven't used them in aggggeeesssss so its possible something reusable has come onthe market.