Quaternions are not a thing in Blueprint, due to Epic not exposing them.
Epic wants to keep Blueprints userfriendly, so they limit it to Rotators (don’t argue about it , I’m with you!).
If you need quaternions and you don’t want to recreate them in Blueprints (which I also can understand),
you might want to use Rotator->Quaternion functions from C++ and do the rest in C++.
If, however, you can’t use C++, then it might be out of reach for now.