Hi Federico,
After reading your question, my initial thought was that I would have expected the promotion of bookmark folders to work exactly the same way as regular folders through Promotion Management in the CMC. However, after looking at my demo system CMC I understand your dilemma. The bookmark folders don't appear to be visible when creating a promotion job. The only idea that came to mind is that since the bookmark folder is part of the Design Studio Runtime (installed by the BIP Add-on), perhaps the bookmark folders can be transported by transporting the Design Studio Runtime as shown in the screenshots below:
I was able to successfully export a promotion archive file containing the Design Studio Runtime but don't have access to another system to test the import in order to check if the bookmark folders are included. Maybe you can try?
Regards,
Mustafa.