Although it was not the intended way of working when the comment file functionality was designed a long time ago, it is possible to have a comment file for, say, and AutoCAD drawing (DWG), using Word (DOC). However, it can require some careful tweaking to get working. I once did an experiment where the External Redline Application (under User Settings in Docman) was set to a script (.cmd file). This script gets a file name sent in. I then had some logic in this script that opened Word. I also remember I had to do some file renaming to get it all to work. The comment file functionality expects files to be named in a very strict way, to work. For this to work at all, you need to have Word files (DOC or DOCX) setup as Document Type = Redline under Edm Basic. That might not be what you want, since you might also use those file types as originals. It might work though, but it was a long time since I tested.
If I find some time I might have a go at this again. If I do, it would be good if you could write more about your exact use case. What files are involved? What applications? Why? How would you use it? Etc.