AP only has a NSEC per server. Users just have a login and password. In theory, a server could use nsec for user login, but the protocol only has servers sign events.

So you would have to run your own server to control the nsec using to sign events.

SSB makes you run your own server as part of every client.

The idea of Nostr is to separate running a server/relay from signing events so that casual users can manage their own nsec without running a relay. But you should still work on running your own relay if you have any tech skills.