I'd like to have a local user in the admins group for when the computer isn't connected to the domain. Currently I use a start up script to create the user and add it to the admins group. Is there a way to use group policy to add a local user to the admins group?
Yes, you can use Group Policy's Restricted Groups to do this. The interface provides options to both look up domain groups, as well as enter non-domain group names.
In this image, local users "administrator" and "isc" are included along with the FCSD domain groups. Note that names are not verified against the local computer source, you must ensure the users exist.
The groups function in Group Policy Preferences will work around the issue identified with Restricted Groups doing a "replace" rather than a merge.
If you can't take a dependency on Group Policy Preferences (e.g. due to client OS/SP level), in you script also create a local group on the machine and put the user in the group. Instead of defining a restricted group for Administrators, define one for your new group and populate the "this group is a member of" section with Administrators.