Yes you can actually. I've made lots of mixing and matching with parts of my texturepack. Just load it into a drawing program, paste in the panels you want to change, save it as an interlaced .png file to preserve the transparency, then plug it back into the minecraft.jar directory.
"The way I see it, every life is a pile of good things and bad things. The good things don't always soften the bad things. But vice versa, the bad things don't necessarily spoil the good things or make them unimportant."