This 3rd party tool will help scan your birdsite follows/followers who have posted their Mastodon deets in their username or bio - generate a CSV file that you can download (at the bottom after a scan) - and you can use the import tool in Mastodon "Account Settings" (Import and export, Import, make sure it says "Merge") for easy adding. Please note that it might take a little bit of time to process if you have many profiles to import.

https://fedifinder.glitch.me/