Do you know how the custom fields are stored? If they are stored in the postmeta table, then they should be available in our plugin. Please notice that there must be at least one record in the database for that custom field for our plugin to recognize it.
If the custom fields are stored in a different way, then the only way to support it, is to create a new column. When you download the ACF integration, you can get a good idea on how an integration addon could look like. You could also have a look at this documentation that explains how to create your own column, maybe this is enough since you probably only need one column.
Creating a new column type
You probably have to write your own setting that will list the custom fields in a dropdown. In that case you could have a look again in the ACF integration were we created a setting that displays all ACF fields using the ACF API. Unfortunately we don’t really have more documentation on how to write your own settings, so a lot of looking to our existing code and the documentation above would be my advice.
If you have any other questions, please let me know through mail at email@example.com.
For now I will close this topic.