At this time we do not have any bulk import tools to add contacts to a list based on ID. We hadn't looked at that previously, majority of requests we get for bulk import enhancements are around how to use 3rd party IDs for duplicate detection and how to get more data in (more custom field options).
To make sure I capture this request correctly, can you share what yo'ure intending to do? What I think I'm hearing is that you are storing the Constant Contact ID externally in your system/database. You want to be able to send a request with just Contact ID numbers to add those contacts to a specific list without sending the email address. Can you share more about why this is important and how you'd use this? Also, if you could correct me if I understand this wrong that would be great.