Add domain user to local administrator group on non domain system running Windows 7

I would like to know how to add a domain user to local administrator group of my system. My system is a non-domain system running Windows 7.

I want to add this user to the administrator group, so that when I use this user to app pool it should work and Windows authentication to SQL server should work.

I can not add my system to the domain, but I do have a domain user with access to SQL server (Windows authentication only).

I can connect to SQL server with Windows authentication by using following command:

C:\Windows\System32\runas.exe /netonly /user:user@domain "C:\Program Files (x86)\Microsoft SQL Server\100\Tools\Binn\VSShell\Common7\IDE\Ssms.exe"

But when I add user@domain to the app pool in IIS, the site does not work, as the app pool is not getting permissions to local resources or something like that.

Thanks,