As far as I know (and my memory may be failing me) you cannot. Instead, in the editor you place the verticees (the red dots) overhead and connect them with wooden boards ... thus, each board overhead must be individually placed. At least, the last time I designed that was how I had to do it.
Sometimes the best thing to say is nothing at all.
It's always better to do this before you start a ride but...to get wooden catwalks, you will have to actually build a piece of track, right-click it and check-off catwalks. Uncheck everything other than the catwalks, and click generate final. When the catwalks pop up, highlight them, and drag them to where you want them. You can then shape them by looking at the outline. You can do this as much as you want until you have enough. [8D]
I hope that's somewhat clear there...anyone else want to translate if it's not enough?
edit - and if you want to delete the track segment, you can, you already separated the catwalks from the track. But if you hit "clear auto-supports" after your catwalk creation is made, because they are considered auto-supports, the catwalks WILL get deleted. You don't want to accidentally delete all of your work.
edit 2 - ... [:I] [:D]
"Don't be a disability"... yep, still keeping that.