I understand you basically want to configure Moodle to use that new Azure database, no? To do this, you don't need to add a new instance to the deployment, why did you say that?
To do so, you just need to access the Moodle instance and configure it to use the Azure's database. The configuration file is inside the /opt/bitnami/moodle directory. Once you do so, you can stop/remove the MySQL instance that is launched in the Bitnami Moodle MultiTier solution.