From the main window, you can use Image | Quick Process | Modify File Info (Ctrl-Shft-Q) to add a comment to multiple files at once.
I haven't tried this, but copying your old-style JPG comments to the Comment field in the IPTC header of the same file should be possible through the Image | Batch Edit Metadata command. For the first (target) field in that dialog, use Description > Comments and with the From button, select File Info > Comments as the source (I'd try this on a few test images first).