I would like to add additional insulation to my exterior walls. What would be the best type and way to do this. I'm currently set up like this. From the inside....1/2" drywall, R13 insulation with foil vapor barrier, 1/2" hard board, 3/4" hard foam, steel siding. I'd like to add an additional R13 if possible without tearing out all the drywall. Is there any way I work over the existing wall, inside or out without any condensation problems?

You could add EIFS on the outside, replacing the steel siding, but you won't get R-13 out of it. Other than that, you have no space in which to add insulation, even if you could manage to get some in there.

You have all you can get without ripping out everything and building out to 2 X 6 walls.
Add insulation in the attic or make sure you have the best windows you can afford to save money on utilitys.