Isso é chamado de roteamento estático. Você adiciona uma nova rota manual na tabela de roteamento dos roteadores. Do manual do DD WRT :
Static Routing
A static route is a pre-determined pathway that network information must travel to reach a specific host or network.To set up a static route between the router and another network:
Select a number from the Static Routing drop-down list. Enter the following data:
Destination IP Address – The Destination IP Address is the address of the network or host to which you want to assign a static route.
Subnet Mask – The Subnet Mask determines which portion of an IP address is the network portion, and which portion is the host portion.
Gateway – This is the IP address of the gateway device that allows for contact between the router and the network or host.
Depending on where the Destination IP Address is located, select LAN & WLAN or WAN from the Interface drop-down menu. Click the Apply button to save your changes. To cancel your unsaved changes, click the Cancel button. For additional static routes, repeat steps 1-4.
Isso é feito na página Roteamento avançado.
No seu caso específico, a sub-rede é / 8 ou 255.0.0.0. Gateway seria o ip 192.168.0.2 da VPN