Currently (as far as I know) there's only two ways to turn Knockback in to Knockdown:
1) The Sudden Acceleration IO. This is a decent option, but comes with an opportunity cost in that a slot must be dedicated to it so you're likely giving up something else (accuracy, damage... and most likely part of a set bonus).
2) The Overwhelming Force IO. This is a good IO in that it both increases damage and changes KB to KD. The downside is that you can only use one of them per character.
For KB heavy builds, neither of these are great options.
So here's my thought... create a new Knockback IO set where one of the set bonuses is KB to KD. The IOs themselves can be the typical ones (Acc/Dam, Acc/Dam/End, Acc/Dam/Rech, Dam/End, Dam/Rech, End/Rech), and the first few set bonuses would be typical as well. But either the third or fourth set bonus would turn that power into KD.
Set bonuses normally apply to the entire character and not one power. So if the above is too difficult to implement, then as an alternative have the set bonus reduce all of a character's knockback by a third. Then once three powers are slotted, all of the character's powers would be turned to knockdown.
I'd love having that as a option on several of my characters.