“additional duties as assigned”

So I'm working a new entry level job which, like many, lists among my list of duties “additional duties as assigned”, so those extra little tasks that no one wants to do can get thrown to the bottom of the ladder where the admins reside for them (me) to do. At this point, my supervisor is assigning me additional duties that pertain to another supervisor's team. By completing these tasks, I would be performing work for another paid position (Admin for the other team) that is not currently filled. Since this is “work being done for another paid position”, what steps can I take to say no to this work, and that the agency needs to fill the position, or to enforce that I should be additionally compensated for the work? Thanks