You have to create a bunch of firewallrules. I recommend using hosts-aliases (like wan1dst, wan2dst,...)to keep the amount of rules low.
Create a firewallrule at interfaces>lan like:
pass, proto tcp, source any, destination <wanXdst>, port <119>, gateway <wanX>.
Make sure these rules are above of other loadbalancing rules. Ruleorder is important to make this work correctly.