As long as it resolves to a string on eval you can put any expression in parens, and you can import external text files like so:
[
(builtins.readFile ./config.exs)
# or using interpolation to blend strings with code
''
some import string content
${lib.fileContents ./other.txt}
''
]
builtins.readFile adds an extra newline at the end of the file, if you need to avoid this you can use lib.fileContents from nixpkgs instead.