You can merge the DSL internet connection by using the same class of IP address that you use with your Head Office network, connect the DSL internet connection to an ISA server and then to you LAN so that you will not face any problems while accessing the LAN or H.O. WAN.
Create a site to site VPN. You will need a VPN firewall or VPN appliance at each end preferably made by the same vendor. The hardware vendor should be able to instruct you on troubleshooting the connection.