I assume you mean an LJ-based site that's not one of the preconfigured ones? I don't know that there's a way to do imports from those, but you should be able to set up crossposting. When you set up your new External Site, choose Site 'Other Site', Custom Service Type 'lj', Custom Service Name (something appropriate), and Custom Service URL 'http://your.site.url/interface/xmlrpc'. Should work with any not-incredibly-ancient LJ-based site.