Group policy - restricted groups. How to specify a -local- user as member of the administrators group in group policy

Hi

With restricted groups I can specify the end user -domain- accounts that are members of the local administrators group on domain PCs. But - I need a particular LOCAL account on all the machines to keep its membership of the local administrators group for testing reasons. At the moment restricted groups is striping this local account of its admin access.

Is it possible to specify a -local- computer account as admin on all the PCs via group policy or it can only be done with domain accounts?
thanks