If you don't have a way to extract the data from the other database in a file, and then reimport it in Campaign, then the only way I see to do that with OOTB features is through the FDA option.
The FDA option in Campaign allows to connect Campaign to an external database. However, this is a paid option and only work with the databases listed in this document under the FDA section: Compatibility Matrix
Sorry but I am a bit confused now. Because in your very 1st post you mentioned AAM (Adobe Audience Manager) which is of course anonymous data for DMP use, it is why I gave you AAM/AC connector documentation.
But in previous post you mention AEM (Adobe Experience Manager) for a form submission, but without authentication. So you use the webCMS AEM for form achievement but not its own data persistence storage.
Anyway, I confirm, you can't use AAM for storing PII information.
You can use the AEM/AC connector to connect information, either using People service, or with the AEM directly connected to AC data storage.
In your case, you can use either Adobe Campaign visitor table (schema nms:visitor) for non optin data capture, or directly in a nms:recipient information is the data is more qualified, or even more a custom schema information if accurate for you.
I mean that with AC classical version (v6/v7); for ACS (standard) version, I think it is only nms:recipient storage.
Campaign info is stored in the Custom Database Table..
A trigger written on Custom Database Table creates a file with Email ID and places on an FTP server, as soon as the Campaign info is stored in the table.
In Adobe Campaign we write a workflow with an activity in the ETL workflow called a File Collector that checks for a new file on the same FTP location. As soon as that file is available, it reads email id and stores in the Recipient Table.
The workflow also deletes that file, so as not to repeat processing it.
The workflow is scheduled to run periodically.
But I am being asked why we need to create so many disconnected processes and stitch them together to achieve this.
Adobe Campaign must have some out of box functionality to import from a Database in to Recipients Table and why am I not taking that route?