I do this on several firewalls. It is pretty easy to do.
First, create the virtual IPs. In my case, I have a /24 that I use most of for a round robin NAT pool. I proxy arp these IPs. The /24 is subnetted into smaller blocks so I can carve out the other IP's I need for other services.
Then just create outbound NAT rules.
Remember to set the pool options in the rule, such as round robin, RR w/ sticky address, etc…



