If you have the option of providing a custom configuration file, I think you should keep it in `/nix/store`. You can make an expression to write the configuration and pass the value in as `-c ${bitlbeeCnofig}`. I do this in my LightDM pull request - [line 54](https://github.com/NixOS/nixos/pull/129/files#L2R54) writes the configuration and I use it in [line 98](https://github.com/NixOS/nixos/pull/129/files#L2R98).