the Add-on you searching for is Splunk DBConnect v2. It gives you the possibility to import/export database information. By default there is no support for MongoDB.
But there seems to be a solution.
Follow this link.
also take a look at that
Oh sorry, i did not know that you have to purchase it. But i looked at the site of unityjdbc. Theres seems to be a free trial version of the driver.
Hope this will help you.