Assign public IP to second LAN

[clandmeter]

I currently have 2 Local networks LAN and INFRA.
I am using proxy ARP to forward some of the ports of one of the WAN ip addresses available to me.
The problem with this setup is, the outgoing traffic is still coming from my primary WAN address, not the proxy arp address.
I have added a 1:1 mapping for the proxy arp IP to my INFRA lan, but if i do that, i am unable to access the internet anymore.

Is it possible to assign one of my public ip addresses to my INFRA network so traffic would come from that address?

[KOM]

Wouldn't an Outbound NAT rule do just that?  I have an Outbound NAT rule to make it look like my mail server is sending mail using the same public IP address as our inbound mail scanner.  I have the public IP address port forwarded to the mail scanner, and the Exchange server set to talk outgoing on the same public IP address that the mail scanner uses for incoming.