To your last question: yes! In a "Nix expression" defining how you build the package (i.e. a "derivation") you can specify where the source is fetched from via any mechanism (e.g. http, ftp, git, etc.)

The documentation is good but it can sometimes be a bit rough. There are some good blogposts on how to achieve specific things that you may find when searching around. Also the IRC community seems is quite active, responsive and friendly :-)