I'm afraid you're out of luck, unless you've got access to a higher resolution elevation dataset. At the moment there's no editor that allows us to edit the .dsf tiles or the source SRTM data for that matter. The only way is to edit the height points by hand, but that's not a simple task in its own. LR says they won't release tools that will allow you to make such edits, and unless 3rd parties come up with something we won't see an editor anytime soon. However, I've read that you can convert the SRTM data into graphic files (GeoTiff ?? or something else I can't remember), and then hand-edit it in a Photoshop or something similar, but this isn't any easier nor a good solution if you ask me. You will have to do some googling on your own if you choose this path, and you will probably need a windows machine as there are no tools available for the Mac if you need to convert the data into graphic files... Not much of a help, but this is the reality for scenery designers atm.