if you put in the catalyst without the hardener you are defeating the purpose. and the paint most likely won't dry properly because the epoxy will never set. as far as mixing epoxy into the paint I can't tell you if that is a good idea or not. The best thing to do is mix a little up and try it on a piece of scrap material that you finish up just like the airplane you are building and see how it looks after it dries. If the paint finish looks good after it dries with the epoxy added then go ahead and do it that way. Only then you will know that it worked and that It will give you a nice finish with the added epoxy protection. I hope this helped.
Best Regards
Nevin M. Levy