I saw mention of specifying a mnemonic to restore an identity, but that seems to be different from an nsec. I can't find any trivial/documented way to give the agent an existing nsec manually.
The Keychat plugin uses a seed phrase, just like the Keychat app, which also uses a seed phrase by default.
https://github.com/nostr-protocol/nips/blob/master/06.md
https://github.com/nostr-protocol/nips/blob/master/06.md