Do you have access to the DSL router's configuration settings? If so, then add a route to the other PCs network.
Let's presume that your PCs have been assigned an IP address in the 10.10.1.x range and the PCs connected to the wireless router get a DHCP address like 192.168.1.1. If the wireless server has IP address 10.10.1.10, then add a route to the 192.168.1.0 network through 10.10.1.10 with a 24 bit mask (255.255.255.0). When the router sees address 192.168.1.x, it will forward it to the wireless router instead of the Internet. Do you also assign fixed IP addresses to the wireless clients?