is it possible to constraint or parent /follow an object into other without changing their current position?

got a character: body and head, I want the head to follow a null(b) that is under a second null(a) (that controls the body) but I want to keep the current layer hierarchy of the main head and body without moving it around or parenting each other.

when I link position of head to nullB and I SRT that null under nullA that controls thr body, for some reason when I move nullA it doesnt move nullA (thus not moving the head).

hope this makes sense