Yes, I think you can put Acting XXX on your resume. Depending on the nature of titles in your field / workplace, you can use it as a position header, or you can list it under the duties of your official title position. E.g.,

Admin Asst.
Responsible for x, y, z. accomplished x, y, z. Also served as Acting XXX, performing x, y, and z responsibilities.

More important than your title is describing your tasks, so make sure you include the tasks of the higher up position that you're doing.

Hi - need some help with language on my resume. I currently hold an admin position for the last few years. The person in the position above me left and I've been doing both roles. on my resume I have my current title but can I put "acting xxx" along with my title to show I am also currently doing the other role? Is there a better word to use than "acting"? Should I just not out the other title at all?