Well, keeping a button pressed reminds me of the charge-up attacks. Go search for that term for a little more detailed explanation
Pressing certain keys right after each other isn't that difficult. It's just quasi-impossible to just edit the damage. You'll have to spawn different frames of ball-attacks in case the ball should have different injury-values. For example, you add in your standing, walking, and defend-frames something like: hit_ja: 290
In frame 290 and following, you add a hit_a: 291. "next" should link to the attack itself. You'll have to have multiple versions of the same attack; the only thing that differs is their strength (in which case you can determine yourself how you want to execute it).
Frame 291 should have a hit_a: 292, next should go to the second version of the move (which should be a little stronger, ofc). Do this for as many differenciations of the attack you like. Just ensure the wait in frame 290ff. is high enough so that the player is actually able to press the attack-button (as in: >3).
'nuff said for now.