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* Get into the MySQL command line utility: mysql -u root -p
* Enter the following commands:
1. Type in: CREATE USER 'kodi' IDENTIFIED BY 'kodi'; and press return
2. Type in: GRANT ALL ON *.* TO 'kodi'; and press return
3. Type in: flush privileges; and press return
* Close out the command line tool with \q
Is actually identical for all the different Linux ones. So if you can connect to your QNAP command line (e.g. via ssh) execute these commands. The root password for mysql is admin unless you changed it.