I've never tried storing onion sets, so I can't say whether you'll have any luck with that.

May I offer another idea? If you have a little extra space in your garden, plant your remaining onion sets. But plant them close together, 1 to 2 inches apart, and in a few weeks you ought to be able to harvest green onions (scallions).

If you're out of space, you could probably just stick the extra sets in between the onion sets you've already planted. As I said above, just thin these out once they've sprouted as green onions, and let the sets you originally planted grow as normal.