Yes, sure. It is totally possible with Ecwid.
In order to do this you should create special script using our Order API
and Instant Order Notification (ION) API
. Using ION you will detect that new order has been placed. After that you will need to get the list of ordered files using Order API. If this list contain the downloadable product, the script should add customer's email to special mailing list.
If you need a professional help with custom script development, I suggest you to contact our partners from Qualiteam. Please, send a request at email@example.com
with project description in link to this thread for the quote.