This is a problem that is commonly asked. The short of it is, you can't. The 'Punch' action is a game inbuilt over a module component.
You could try (depending in your skill (even then it would be questionable)) do a test to see if the player has pressed the attack button. If s/he has, then make it test if they are armed. If they are armed with your "itm_punch", then make it run the "punching" animation over the one handed animation.
I am not saying it will work, as I have never tried it and this keeps being pulled up in the forums. But it is always worth a shot! Good luck!