You'd probably have to modify the core code to this and end up with all menu links opening in a new window.

You could use the "Links" module to create a link to the other portal that would open in a new window, or use a "Text/Html" module and manually code the link with target = _blank.  I think that's probably the only way around it.  Not sure what House Menu would do, some people highly praise it for it's functionality and maybe that has a way to do what you want, I'm not sure.  I only know you won't be able to do it within the default DNN menus.

Rick

You can use javascript to open a menu link in a new window.

In the Page Settings, go to Advanced Settings -> Other Settings. Change the Link Type to URL. Then use this javascript code to tell it where to go:

void window.open('http://www.externalsite.com');