You just need to do that with an attribute in your characterBP. Add a .kneeling attribute, toggle it on when you hit the kneeling key, maybe with some checks if your character can kneel (i.e. isnt moving).
Then in your animBP you just make a kneeling state that loops the kneeling anim. You can also do it with a 3state system where you have the standToKneel state that plays the transition animation, then it goes into the loop state until your kneeling variable is off, then you go to last state plays the kneelToStand anim.