Generally, you can assign additiona IPs as additional addresses.
To do it, first make ARP publish at Intrefaces > ARP over WAN interface.
After, make route core/additional_ip/0 at Routing > Routing tables > main. If additional IP is not from main WAN IP subnet, also make route wan/additional_net/100.
Sometimes, when separate interface is necessary, together with L2 managed swith you can assign additional IP addresses to VLAN intrefaces.