Using left over pickle juice to make more pickles
I tried it once. They just spoiled in a few days. I think you need to go through the whole pickle-making process rather than just adding cucumbers to a jar of brine. There's a sterilization that you're not getting, I think.

I have seen recipes and pre-packaged "refrigerator pickle" mixes, ("Mrs." somebody... comes to mind) but haven't made any. I've eaten them, and they were pretty tasty.