No.
D&D does not specify why a certain attack misses.* When you make a melee attack that falls short of the target's AC, you cannot be certain that the target's armor is what foiled the attack. It could just as easily have been the target's dex bonus. Or the target's natural 10 points of AC.
If you are holding a touch spell, you can either:
A. Make a touch attack against your target's touch AC. If you hit, you discharge the spell, but do no additional damage.
B. Make a standard melee attack against your target's full AC. If you hit, you do natural weapon damage + discharge the spell.
*Edited to add: The one exception I can think of are cover bonuses, which are treated as an "additional" AC value. However, other bonuses, when present, have no cardinal order in which they are applied, and therefore my point remains. You can't be sure why a melee attack failed, and without that, you can't say that the same attack equals a successful touch attack.