Pretty common occurance from what I can tell. Not sure what the pro's do but my guess is they cut and replace. If you are trying to salvage the spokes be careful that you do not stress them to the point where they are a danger to you when you ride. The fact that they are in so tight may already be a sign that you need to replace. I have however successfully removed frozen spokes by doing the liquid wrench thing then using a spoke driver from one side and a plier to hold the spoke close to the nipple so it does not torsion when you give the nipple a twist (did I just say that? My Bad). Anyway this will loosen a stubborn nipple. If you need to go much more beyond this the spoke will probably snap anyway. Good luck but remember it is cheaper to replace a whole wheel of spokes then one visit to the hospital.
Ray Homiski
Elizabeth, NJ