Perform admin tasks (e.g. create database, add diagram support) in SQL Server when not connected to domain
Another solution is to create an SQL Server Log on and give it sysadmin permissions.  This avoids using windows entirely for authentication.  This is what the SA account on SQL Server is, a special SQL Server log in (which i normally disable log on once installed and another admin user is created) with sysadmin privileges.

This would probably be the best bet as it does not require creation of a local user on your machine and having to run SSMS in a different manner every time, just select Sql Server authentication when logging in and use the new credentials.