If you want to make traffic appear to come from a WAN address other than the primary WAN interface address this can be done by creating a one-to-one NAT rule.
A One-to-One NAT rule will force all traffic that comes from the LAN address (192.168.168.10) to use the WAN address (205.100.100.10) when going out to the Internet.
This diagram demonstrates what is required to force traffic out a specific WAN address.
Article ID: 33, Created: February 19, 2015 at 9:05 PM, Modified: February 24, 2015 at 8:34 PM