You can't do it by parenting, that's for sure. You can only parent a layer to another layer: that's it.
If you currently have keyframes for the emitter position, think about what you'd have to do: you'd have to have two different expressions, one for each keyframe, linking emitter position to the nulls' positions.
So you'd not only be trying to make TWO expressions for one property -- a big no-no -- but by linking the first motion path keyframe to a null, you'd be constantly revising the emitter position's PAST position... and particles have a lifetime. So you'd want them to be in two places at the same time.
I really think it's time for Plan B.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA