Of course there is. The only thing that Photoshop can’t do is make a rocket launcher out of some kite string, an empty toilet paper roll, chewing gum (spearmint), a pocket knife and some duct tape… and that’s what separates Photoshop from MacGyver–and nothing else.
Basically, it’s really simple. All you have to do is draw/paint directly on your spot color channel, not a layer. It’s pretty easy, albeit not as easy as layers which is why you should be fairly profecient with your CHOPS. The hardest thing to remember is that you are painting spot color with values of black, and erasing spot color with white. Change the percentage of black, and you change the opacity of your spot color. Screw up, or don’t like what you did? Use white to erase the spot color.
HTH