There is an example of proxy arp configuration in the arp man page
But I would advise to get another box that you can equip with 3 NICs (external, DMZ and internal)
If you have servers with a public address in your local LAN, those servers, if compromised, can be used to launch an attack against the complete local LAN.
By placing the servers/hosts inside a DMZ these servers still could be compromised, but not used as a base for attacking your internal network.