iamdanieljs on "Import users from non-wordpress database"

I have a custom-made script, running a bookingsystem. I would like to have all the clients at the bookingsystem, automatic imported to my wordpress site as normal users.
At the bookingsystem all users are registred with a username, email, frontname and lastname in a mysql database.
Does a plugin with that ability already exist? I have been searching for hours now, but without luck.