To do so go to your DW feeds page (Settings - Read - Feeds) and in "Feed URL" window enter http://lj_username.livejournal.com/data/rss (replace lj_username with the actual username of LJ blog you want to subscribe to).
After that DW will display another page prompting you to enter a name for the feed. Enter it, and then subscribe to it.
If someone had already subscribed to that LJ feed before, DW will display an existing link. Click on it and subscribe.
You can subscribe both to personal journals and communities. Naturally, you will see only public entries, but it's still better than noting.