What you do is go into the html in a text capsule and add target="_blank" to the link's code. I can't type it exactly here since html won't show up in an answer.
But hold off on this. I just read this morning (by a Pcunix comment on a Mark Ewbie hub) that Google may consider your page abandoned after it has been left for a given amount of time and its bouncerate would be no different than if no time was ever spent on your page.
So apparently if someone spent a lot of time reading your hub, clicked on a link which opened a new window, Google doesn't have an exit time for your page (since it is still open). I have asked Pcunix a question about this and will let you know what I find out.