At this time, there is no easy method or protocol in place to send data from AtoM back to Archivematica. There is certainly potential for future development, but I would expect it to be a major project.
First, newer versions of the SWORD
protocol (the v1 protocol is currently used to help pass DIPS from AM to AtoM) can include bidirectional messaging, so that would be something to explore.
More importantly, i think that some of the ideas about the Preservation Action Registry (PAR) would have a place in this. Some links in case you are not familiar with this research project:
The broad idea is to abstract Archivematica's Format Policy Registry into something standardized that can be used by many different applications, for handing off preservation workflows from one tool or application to the other.
Currently there is a lot of duplication of effort even between AtoM and Archivematica - when a DIP object is passed to AtoM for example, AtoM will still generate an access reference display copy - often larger in size but worse in quality/resolution than the original access object included in the DIP - instead of simply reusing the DIP object. As you note there is also no bi-directional communication either, so this is a brittle point in the chain. AtoM's access derivative rules are also not configurable at present without making modifications directly in the code.
There are also some debates within the community about whether an AtoM to Archivematica workflow would be the best idea, in terms of digital preservation best practice. AtoM is NOT a digital repository, and is not suitable for long-term preservation - it is primarily an access system, so storing your original, high-resolution master digital objects in AtoM may not be the best approach.
If there were a full 2-way integration between AtoM and Archivematica, facilitated by both projects using PAR, then I would think that if you started in AtoM and then passed content to Archivematica for preservation, perhaps at some point the original masters should no longer be stored in AtoM (i.e. once you have an AIP securely stored). Instead, AtoM would point to the AIP (in whatever repository or system it is ultimately stored), and maintain only access derivatives, based on whatever rules you had set in PAR for AtoM. This would avoid duplication and storage bloat, as well as ensuring that AtoM is not considered the system of record for preserved digital objects.
These are just my ideas though - members of the Archivematica team will have their own take, and surely our community will have many other ideas.
In the meantime, you would need to move content manually between the applications.