Have a look at the Bsky API and add the possibility to import your posts.
1) Create aa connect button
2) Use a modal that prompts user to add an App password on https://bsky.app/settings/app-passwords
3) Save username and encrypted App password
4) Use encrypted password to make requests to the Bsky endpoints regarding user posts and the rest.
See https://atproto.com/specs/xrpc#app-passwords