Simple answer: yes
Complex answer: KD is the same thing as KB, just very low mag. The break-even point is somewhere just about 0.7 (probably around 0.72 or something). So, if you enhance Repulsion bomb enough that it turns into KB of mag >0.72, then it will be knockback, instead of knockdown.
This will also depend on the level of the enemy you're fighting (the higher the level, the more the mag is lowered), and the type of enemy you're fighting (clockwork are weak against KB). So keep that in mind, too. Basically, if you're fighting higher-level enemies, you can get away with a bit of KB slotting before it turns into KB.
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted.
~Ralph Waldo Emerson
"I was just the one with the most unsolicited sombrero." - Traegus
Pretty much what I thought. Thanks for the longer version as well, it's good to be up on such things. 
Very quick question: my defender's Repulsion Bomb does knock-down. If I slot an IO set in it that enhances knock-back (such as Kinetic Crash), does the knock-down turn into knock-back?