Officially you can’t without API. Unofficially the scraper could login with the users password and send messages on the other platform, as long as there is no 2FA or captchas in the way.
As fast as I remember Facebook did ask for the users Myspace password to “import” their data.
Doesn’t matter. As long as sent and received messages are shown in the accounts inbox you can parse them back out.
But how do you then reply to those posts back into the platform of origination from the outside platform?
Officially you can’t without API. Unofficially the scraper could login with the users password and send messages on the other platform, as long as there is no 2FA or captchas in the way. As fast as I remember Facebook did ask for the users Myspace password to “import” their data.