As I understand it a lot of the heavy lifting on that (the Silmarillion) was done by a young Guy Gavriel Kay. But neither of them have been very forthcoming about the collaboration.
It has been years since I read about Kay's role, but IIRC he wrote only a few very brief passages and the pair were open about their roles. I remember being surprised at how little new material had to be written to make J.R.R. Tolkien's own texts coherent.
My understanding is that The Silmarilion was originally conceived as wrapped in Bilbo's Translations ftom the Elvish*. Christopher Tolkien said that he regretted not having that in the published version.
