If you only want to copy the usernames and date of registration then it should be some fairly easy SQL manipulation to achieve it.
If you want to keep the old passwords working you might want to have a look at my Old Passwords extension. Not sure what hashing wordpress is using so it might not be compatible yet.
If you're looking for somebody to to it for you then you may contact me here https://clarkwinkelmann.com/flarum ?
