Is this possible? Or do I have to attach it to one of the surfaces that I've made that lies on this reference plane?

I'm doing a machining from a pre-machining (not a casting). Datum A is a hole center (from the pre-machined part) and Datum B is an offset from this datum, so I made a plane to represent datum B, but can't attach a DimXpert datum to it. I've renamed the plane to -B-, but wanted to add the pretty lil datum flag.

Thanks!