Good question. Mervis natively does not support logging data to external databases, it mainly supports its own DB servers.
BUT that does not mean it's not possible to do. Actually its pretty easy - https://kb.mervis.info/doku.php/en:mervis-db:10-api
This link describes API, how to Mervis RT access the Mervis DB but you can create the same API server implementing just the saveData function (eg. in PHP), point Mervis RT to this server and do whatever you wish to do with the data.