Dynamically assign outbound network interface

I'm establishing a network connection with wvdial (PPP) and I want to assign this as the outgoing network interface AFTER it is established.

The connection is being established by a bash script and after it is established I would require it to change the outbound interface.

The current network interface file looks like this:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo eth0
iface lo inet loopback
iface eth0 inet static
        address 192.168.1.123
        netmask 255.255.255.0
        gateway 192.168.1.1

#auto lo usb0
iface usb0 inet static
    address 192.168.7.2
    netmask 255.255.255.0
    network 192.168.7.0
    gateway 192.168.7.1

As you can see the PPP connection is not displayed anywhere.

How can I solve that?