First off, I've moved this over to Server Administration forum where hopefully someone has useful input or a workaround.
While I haven't heard of adding users from MySQL, you can add users using Tabcmd and csv files. My thinking is you may be able to funnel the info from MySQL to a csv, then create a script that adds the user from the csv file. Once that is created, you can simply set the script to run on a schedule (such as every morning). Here is some more info on the second-part of my idea:
I hope this helps!