Sharing Actors between Levels

I have an actor that has a variable type of other actor. My actor A is in X level and variable actor B is in persistent level,when I want to assign actor B as a variable for actor A, I cannot select actor as variable, only when A and B are in same level I can assign variable.

Here my variable name is floovar, but when I choose the variable, it cannot be set.
So here is my question, how can I be able to share an actor or give as a variable to another actor in different levels?
I know we have a method : get all actors of a class, I can use it to take an actor from another level. But are there any other way to take an actor ? (for example I have a main actor in persistent level, so I want actors in all levels to know main actor)