Hi!

yes, it can be held, but basically, you'll need a move for every "power level" of the last punch... for example in dragon cannon, you need 3 punches that can chain consecutively, each one it's own move, and in the end, a fourth punch that is actually a "charge" animation, recoiling strength to release the last hit, and lastly, the actual attack, or in this case attacks, depending on how much time you keep the button pressed.

So, you'll have to make for example three or two attack animations that require the recoiling animation to be called, and it's command must be the same punch button, but not on press but release.
so,  in your charge animation you must put in the chain moves section different frame windows, during wich if you release the button you'll call one attack if you release too soon, or if you wait for the entire animation to play, the strongest version let's say.

hope it helps, if it's a bit confusing, i can use an example from my game, in wich i have a move that does what you require.

Greetings!

even slope wont work... may be UFE dnt use rigidbody physics for gravity thats y.