So you have two seperate networks that you want to reach your private leased line (I take it this is like a point to point line connecting two offices) and a broadband Internet Connection. There are several ways to go about bridging the two networks together using routers other equipment but in your small networks case I would do this:

Connect your leased line router to your local network giving the router a local IP address that can be reached on your other network, for example if your broadband network is 192.168.0.1 you should make the leased line router 192.168.0.254 or something along those lines. Then in your Internet firewall or router you will add a route. If the network on the other end of the leased line is 192.168.1.1 then you can create a route pointing to 192.168.1.0 255.255.255.0 to the router on your local network and have it push the traffic to where it needs to go.