Open the gunspec.txt file and add melee key = xx (56 is the left alt key, for example), and add the animation frames in the HUD animation frames list like this :
melee start = x,y
melee end = y,z
where x is the start frame, y is the frame where the weapon makes contact, an z is the end frame.
I'm not sure if you can add more than one extra animation, though.
