My short answer is no.
It is determined by the lookup used for the location attribute. The location attribute of the project object is OOTB and therefore you cannot change the lookup associated with it.
You cannot change the lookup associated with an attribute.
You would have to delete the attribute and recreate it and associate it with an OBS Browse type lookup.
That would make you to choose the OBS (Location) first before you can choose a unit.
You cannot delete OOTB attributes even if you wanted to do that.