If you already have a file, that gets much trickier.
The following requires knowledge of XML files. Before you do the following, I’d strongly suggest marking a point in project history in case something goes wrong.
To “merge” the two files, you can open the file from the source project and copy everything inside the <CommentList>
element and paste it at the bottom of the <CommentList>
element in the file from the destination project. That should be safe to do.
Also, I forgot to mention before that any note tags being used in the source project will not carry over to the new project and the copied notes may take on some of the note tags being used in the destination project if the internal IDs match.