You can't really "join" an IP only game but there are some tools out there to help out.
There is another tool called dfpinger (not sure if it is in the downloads section, probably should be
) that you can join by ip.
I saw on the DEM forums that NedFox is going to release an update to the BHD Join tool to support ip joins as well as favorites listings and some other stuff.
NL blocks the usage of IP joins to try and keep people from hosting 50 players servers (my opinion only).