Not sure if it is the same on Fortigate_VM, but on the hardware boxes you can configure a "secundary ip" address on the interface.
Go to "Network > Interfaces" and edit the interface, at the bottom of the page you should have a check box "secundary ip address" if the interface has a manually assigned ip address. In the box that appears, type in the new ip address for your FGT, only 1 address is necessary..
- outbound connections will by default take the wan interface ip address for natting. If you need to access the internet with an address from the new ip range, you need to create a "ip pool" and use this pool as NAT ip on your internal -> outside security policies
- if you need inbound connections on the new ip pack, you need to configure vips for these addresses/ports and use them in outside -> internal policies.
No it normally isn't necessary, the ISP will use the original ip as path to the outside world as they will also have configured a 2ndary ip on their box. So you would use only the original default route that was already configured.
Having 2 default routes with the same metric would also put your FGT in "load balancing", sending packets out with source address 184.108.40.206 for 1 packet and 220.127.116.11 for the next. It would cause you troubles with outbound mail etc where the source ip is checked.
You would need to a a 2nd default route in case you have this setup: