NixOS is a general purpose distro (I use it on my router, server and laptop, and plan to install it on my phone, it doesn't get any more general purpose). To run packages that aren't in its repo, you write a package yourself. Note that unlike on traditional Linux systems, there's essentially no concept of "installing" packages. Packages are built and put into /nix/store, then you can optionally add them to your system packages or user packages and they will be symlinked to /run/current-system/sw or ~/.nix-profile, but there's nothing preventing you from just using the package without adding it to system/user packages.