Interesting. I don’t think that’s possible, but you could put a link from each document’s annotation file to a common file that’s your shared “annotation”.
You might also use tags.
Let’s say you have two (or ten) files that need to share a common annotation. Tag them with something related to the annotation. For instance, tag ships with a tag for North Sails, a sail loft of some renown.
Put a common annotation on the North Sails tag, not on any document. Any new document needing to share that annotation can be tagged with North Sails.
By annotating tags instead of the document you could have multiple annotations per file, each potentially shared with other files.
Just a thought.