Moved to custom coding, I'm not aware of any out of the box solutions.
To fully merge you are going to need new user ids, new topic id's, new post id's, new blah,blah,blah id's and change all the references for them in multiple tables. Then there is the issue of links in posts to topics, posts and attachments and conflicts with usernames.There is script here from 2019 but that is probably going to need some updating but it should work for starting point.
viewtopic.php?p=15314916#p15314916
If you are looking for minimalist approach. look at posts table and there is column for post_username which is going to be blank if it was registered user that posted it. That's used for usernames of guest posters.
You should be able to set the post_username to the registered username, set the poster_id to 1 and increase the post_id's to number higher than your current forum. Repeat for topics and attachment table. Lastly you'll want to increase the ID for posts, topics and attachments referenced in posts.
This would also give you path to restore the posts to original poster once they signed up on your forum as long as they used same username.
Just throwing out some ideas that might work. You'll have to fill in the details yourself.Test, test and test.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison