You only need select the Spaceport and rightclick on it, or in empty space.
It's really strange that you have to do this though; you can't retrofit in it's own screen or under any other kind of selection.
You know, it's kind of funny, but after posting this question, I needed to do this again, but I couldn't remember what the answer was. I knew you'd told me how to do it. I knew that I'd done it before. But I searched all over the game, trying to remember how.
Eventually, I remembered about selecting the spaceport and right-clicking on it. I do that all the time when it comes to ships, but it just doesn't occur to me with bases. At any rate, it's not intuitive, not for me.
But at least I remembered before I had to come here and read your reply again - or post the question a second time. Now that would have been embarrassing! :)