6350 8.4 train, need additional media port for NATted connection

We are running HA 6350 SBCs on 8.4.0p13A.

We have a connection to an outside vendor that runs via VPN. We have an existing SIPREC configuration that works using an SPL option line in the SIP-Interface config, thus:

    spl-options               HeaderNatPublicSipIfIp=10.101.119.5,HeaderNatPrivateSipIfIp=167.69.188.117

This works because the existing steering-pool IP is the same as the SIP-interface IP.

We've run out of media ports on the existing IP- it's an extensive SIPREC flow.

We need to add a second steering-pool IP to handle the flow. We need to apply the NAT so that the SDP reflects the external IP (new NAT pair) instead of the internal pair.