Reading the DMG closely, there's actually a question on this, but the standard interpretation is that a Spell storing item holds a spell 1st through 3trd level spell, and if you hit you get to cast it on the target simultaneously. And then the item is empty, and you need to store another spell in it.
If you can't cast spells while you posess the thing, you can't refill it. In fact, I'd probably rule that you have to "possess" it in order to refill it. Which means after it's creation, nobody can ever refill it again! Talk about useless. Just buy a darned wand.