Not sure why you would want to do that to be honest - not getting what that buys you at all.. But sure go to outbound nats and change to hybrid or manual mode and create the nats you want.
To be honest I think I have gone over this sort of thing before when user needed to source nat from their openvpn connection, etc.
I am on the road for work, and be much easier to put together pictures and how to do it between my multiple segments when I get home - if you can wait til say tmrw morning when I should have some time to do this I can put an example of natting between local segments. But I still don't see the point?? What aspect of natting are you trying to simulate to the internet? To test what exactly?