Currently, you need to start with SDL to create federation because one part of the federation responsibility is to be able to provide the SDL (the Apollo Gateway requests it, then parses and validates it). Anyway, if we could convert the EDN format to SDL that would be a viable path; I started down that way, but found I had too little time to pursue it and for our project, starting with SDL was necessary in any case.

Others have mentioned attempting to generate reasonable SDL from the EDN format, or (better yet) from a compiled schema.