At the moment you can put a fully qualified domain name in the force IP box, and then it is resolved to the IP it addresses when you click OK/Apply.

What would be good is if the FQDN stays there, and is looked up every time you load up limewire. This would be useful for people that are behind firewalls with dynamic IPs, but that always have the same domain name (using a service like dns2go - http://www.dns2go.com).