Let's say that you are accessing a friends network with your 770 and you would like it to route packets to and from your laptop via the usb-network you have already set up.

The precedure is a bit tricky and would be simpler to do with iptables and NAT.
Your friends network uses the network 192.168.1.0/24 and your usb-network uses 192.168.2.0/24. On your N770 you would have to enter the command "echo 1 > /proc/sys/net/ipv4/ip_forward" to enable routing.
On your friends router you would have to set up a new route to your usb-network so that your friends router knows where to send packets adressed to your usb-network. Hopefully your friends router does support this. If it is a linux box then one would enter the command

"route add -net 192.168.2.0/24 gw <ip address to wireless interface on your 770>"

example
route add -net 192.168.2.0/24 gw 192.168.1.16

It's a bit hard to understand if you do not know about routes and how the IP technology works.

Using IPtables and NAT you wouldn't need to set up the route on the router. But you would have to configure iptables. But there are good guides for that.