Maybe matinee or maybe more like a Timeline.
If you do a timeline with a location offset vector track, then when the timeline is ended (use flat tangents BTW) set the actor’s Z velocity to something specific. So it makes it look like the timeline is still playing as the character continues off in the same direction as the last part of the timeline. OR you can make the timeline really long and just have the initiall knockback-falling part go until the point where they would reach ground level, and then have it keep going down, down, down for like 200 seconds at the same angle. When the character hits the ground it can tell the Timeline to stop playing and resume normal control, stun/lying on ground/get up/ whatever.
Might want to use 2 timelines actually, one for horizontal movement (knockback) and one for vertical, so you can stop the horizontal one when the character hits a wall or another character, and stop the vertical movement one when he hits a floor.