How to add a local user in the restricted administrators group?

hello all

My objective is to restrict membership to the local administrator group of a computer such as domain admins and administrator , however I like keep a local user such as desktopadmin local account to the administrators group

Using group policy :When I try to add a local account such as desktopadmin( not part of the domain)  to the restricted administrators group , i get a window shows that I can only add domain user or domain groups! How can I add a local user that had been created as a local admin in the computer?