Frozen nipples? (ha ha ha)...I had a rim the nipples were so frozen vice grips and a torch wouldn't help. I tried using 2-3 different rust busting oils including PBB to no avail. I ended up cutting them all out with a 4" grinder.
So I cobbled together a contraption to loosen those stubborn nipples. I had this old electric rotisserie that I was to lazy to clean and was just going to throw out...I took it apart and made an adapter to attach to my truing stand. So, if you can visualize, with my rim mounted on my truing stand, I turn it on and this unit will now turn my rim nice and slow. I put a troff (sp?) under the rim and fill it with PPB/kerosene mixture. So the rim rim turns slowly through the mixture coating all nipples continuously. I let this run for 24hrs. Then they all come apart like butter.
If you absolutely have to reuse old spokes, at least get new nipples, esp if they were originally alumin. because the threads stretch and will never hold right again and also makes them more prone to cross threading or stripping.