Nat,
I have been told that there is a way to do this. Apparently there is a wax seal in there somewhere (this is from another snow globe collector). He said you can use a needle and syringe filled with distilled water to inject water into the globe. Then you use a hairdryer to warm up the wax seal and re-seal it.
I have never tried this, and have been afraid to take any of mine apart. If anyone has an old globe that they don't mind taking apart, and they test out this theory, please let me know. I have a few globes that have some bad air bubbles in them now.
Sue Ellen