You can use port forwarding in iptables, to forward a port to private ip , here is how you can do that :

iptables -t nat -I PREROUTING -p tcp -d <listenip> --dport 22 -j DNAT --to <ipToForwardTo>:22
iptables -I FORWARD -p tcp -d <ipToForwardTo> --dport 22 -j ACCEPT