I ran into the same problem/question a while back when I was trying to create some custom symbols for LAN diagramming.

So far as I was able to figure, there's no way to do it short of collapsing a rectangle down to look like a line.

I didn't really like that solution though.

FYI, I'm using the latest version of OGpro these days and connecting lines to lines is an option that's turned on (default on mine) via the Properties:Connections Tool checkbox called "Allow Connections From Lines".

Maybe there's an analagous option in your OGv3 that we both missed, or maybe time to upgrade to 4 or 4pro? :)

Good luck!
-Matt