From Tobias in slack:

We have a customer who uses the external dam module. The customer would like to migrate to 6.2 but had to realize that this module is no longer supported in this version. Apparently such customers get the dam connector pack for free. Unfortunately the customer does not use bynder or amazon S3 as external DAM. He uses https://www.widen.com/. Is there any documentation how to migrate from the external DAM module to our connector pack. Or if not, is there any documentation on how to connect an external dam system that is not supported out of the box? The documentation I have found so far is very poor.

After chatting with Tobias further on slack, the issue is that there is no documentation like https://documentation.magnolia-cms.com/display/DOCS62/How+to+configure+a+custom+e-commerce+connector

After further conversation with Ilgun, the addon-produced external dam makes use of the mgnl DAM API to connect external systems S3 & Bynder. If a customer wants to connect to another third-party tool, they should also make use of the DAM API. Therefore, in terms of documentation, this info is not missing from the external DAM connector pack but rather from the core DAM documentation. 

The DAM API doc is here  https://documentation.magnolia-cms.com/display/DOCS62/DAM+API and very briefly mentions "Capable of connecting to external asset providers and other systems." It seems that the whole page has not been updated in a very long time.

This page should be reviewed as part of MGNLDAM project and possibly extended to better showcase the possibility of connecting to a third-party DAM solution.