Umm, if I understood the question correctly, then this might be a good answer...
If RollerCoaster Tycoon 3 textures has the file type .OLV, then they might have been compiled from an other kind of picture file.
(Like, Source textures are first compiled from .TGA)
If you figgure that out, you might be able to find a decompiler.
After decompiling it, you have a useable picture file which paint or photoshop can handle.
Then, you put it into a .WAD.
Simple if you find a de-compiler, and if that's the way the textures was made from the beginning.