Communities can't cross-post, unfortunately. The way I've worked around it for a Dreamwidth comm I administer is to set up an LJ subscription to the Dreamwidth comm's RSS feed, but that still requires people subscribing to the RSS feed here rather than continuing to read the (now-abandoned) LJ comm pages.

There may also be a way to use a combination of IFTTT and LJ's post-by-email feature for this. See this kind person's comment for more info.