WSUS and Domain PCs on a Different Subnet

All servers are Windows 2003 SP1.
All workstations run Windows XP sp2.

I have an in house WSUS server at our main office. I have other PCs on different subnets that are part of the domain and authenticate back to the corporate office via hardware vpn tunnels. How can I add these PCs to my WSUS server so that they are updated?