OK. The way Pagerank works is a pageranked is assigned to a specific page or URL. If for example my pagerank 6 blog is linking to yoursite-dot-com/yourpage.htm Then your page gets a pagerank 5 or something similar, and then every page that yourpage.htm links to gets a derived value from the pagerank 5 on yourpage.htm. maybe pagerank 3. So - to answer your question. If your main domain has a pagerank of whatever, you can perhaps sneakily edit your index page and add a little php so that index.php is just a front-end for other pages. This only works if your domain name has the page rank though and I'm not sure it still does work.
...Or you can 301 redirect the PR domain to the site you want it redirecting to. That should pass over a lot of the link-juice!