Big Finish took a stab at "adapting" it. It's one of the few episodes that was never turned into a Target novelization (due to, I expect, Douglas Adams being a "proper" writer), and hasn't really received a full treatment...until now.
With the recent release of Gareth Roberts' adaptation of Shada (available in hardcover), Shada receives a loving, respectful treatment, and the book's tone reads like a lost Adams manuscript - witty, funny, and easily surpasses the "original footage", creating a very splendid reading experience.
(In fact, you can read some sample text by clicking this link).
One could easily see Adams writing this adaptation, but it's more than just Roberts doing a good impersonation - as he explains in the afterword, Roberts built upon Adams' original work and notes. (Shada was not one of Adams' favorite stories, with Adams downplaying the VHS release, claiming that he "didn't know what he was signing" when he permitted use of existing footage). This is, quite simply, a charming must-read if you like Adams or Who.
Full of nice asides, references to both past and current series, Gareth Roberts' Shada has finally done right by this story. A must-read.