It will still be on your system until garbage collected, so if you decide to install permanently, it will be instantaneous. "Permanent" is a fuzzy concept with Nix in any case, as all it means here is that it's in your path and won't be garbage collected. In this case you just add the package name to an array in your config. It's extremely simple to do, just editing a list in a text file. I understand you want to just run a command, but perhaps Nix is not for you if easy command line usage is more important than automated reproduceable OS config.

That being said, I think there are indeed ways to partially manage your config using simple command line commands but I haven't found the need myself.