Nix model is that you work with expressions that allow to build packages from source. Binary packages are provided in format "if you performed build described by hash X you would get the contents of this archive".

For proprietary software, you have to consider the binary as-distributed "source" and build a version with proper dynamic linker an library path. Some third party binary wouldn't run as-is on NixOS, but there is a package that sets correct linking settings post-factum.