|And your question is? (sorry, we're not mind readers)|
You would need to set the LAN side of the router to an IP in the same subnet as your existing network. Somewhere in it, you'll have a DHCP server setup. Check it's settings. The LAN side of the router will need an IP that's outside of the DHCP scope so as to avoid duplicate IP's. You will want to disable DHCP on the router
Client's needing external access (internet) will have to have the LAN IP of the router as their default gateway IP.
DNS settings will be tricky, if you have a domain, you will want to forward your DNS to your ISP's DNS server addresses. All clients will have to point at your DC (with the DNS running on it) as their DNS address.