This is a somewhat tricky workflow, but I think it's probably common enough that we should have a better way to address it. I believe we usually suggest that any detailed correspondence from author to reviewer be attached to the revised document itself (for example, as additional attached pages); or attach the revisions as a supplementary file (though care will have to be taken that this file isn't subsequently published along with the completed submission, which can conceivably be the case if you have configured OJS to publish supplementary files, and if your layout manager doesn't delete the supplementary file during the layout editing process).
In the meantime, I've filed a feature request here
. Please feel free to CC yourself to that report for future updates, and also add any further comments as you see fit. As I mentioned in that report, I do believe that this kind of functionality is or will be present in OMP, and so it's probably just a matter of time before it is available for OJS. But it's still worth adding as a request.