Planning Forms - User Variables Selection Menu

I am developing some forms using Planning 11.1.2.2 and have a question about user variables.

Is there a way to only have certain members available for a user variable? My question comes largely because we have multiple hierarchies.  We have our standard Entity hierarchy as well as an alternate that is comprised of shared members which are a subset of the primary hierarchy.  The form I'm building needs to only show the shared members under some sections of the alternate hierarchy.  More specifcally, we want users to be able to see in the rows of the form all of the level zero members under a certain SBU which are are also shared members in the