I wrote a nano tool that tries to extract the #Fediverse accounts of your #Twitter followings: https://fedifinder.glitch.me/

It searches for the patterns @user@host.tld, user@host.tld and host.tld/@user in the screen name, description, location and URL field. It displays them to you in the correct format for easy copying as well as a CSV download that can be imported to #Mastodon.

Known issue: Mail addresses are included as false positives because people write their handle without a leading @: name@host.tld