I think it's as "simple" as with Bitcoin. You get a 256-bit hash from your nsec, then run it through some sort of cryptographic function, and from that you derive the mnemonic list of words using BIP-39.
Discussion
What I've never seen is the opposite -- what OP is mentioning here about using a seed phrase with an existing Nostr app. But if you have a seed phrase, then you can revert the process and get the nsec. Has anyone put together a tool for that? No idea.
