You can try using a penetrating oil, like some of the lubricants meant to help unsieze car engine bolts; there's usually enough airspace inside the threading for it to run down in, if corrosion hasn't blocked that (and if it has, you probably don't want to reuse them anyway).
In my experience with taking apart many scrap wheels to save various parts of them, if the nipples are siezed onto the spokes themselves, then there is no point in saving the spokes as you will probalby damage the threads or the spoke head at the other end in the process of trying to get them loose, either on or off the rim.
I've dremelled out the nipple heads, so that I coudl get the spoke off the rim, then clamped the spoke in a vise and used spoke-wrenches or vise-grips on the remainder of the nipple, and usually the spoke gets twisted and damaged in the process, or the threads get crunched.
If I can't even turn the remainder of the nipple, I have tried to dremel or file it off, and again, the threads get damaged. Once they looked just fine, but when I re-used the spokes later, the final tensioning stage of truing just started shearing the threads off.

I can imagine what would have happened if I had a front hub wheel built out of them, and they'd just barely survived the tensioning....
So be careful if you do have to use extreme measures to get them off; you might wanna keep an eye on those spokes before and after every ride.
