So, it's the same domain but new site structure/underlying CMS which has different URLs? I don't quite understand how you have multiple sites, vs multiple pages, but that's because I'm reasonably unfamiliar with DNN. Or am I wrong and you've consolidated multiple domains under a new URL?
Assuming 1 to 1 mapping of redirects is out, are there any opportunities to map folders? As in anything under site/folder/pages is mapped to the new site/new-folder? It's not ideal but gets them one step closer.
Alternatively you should make a very good 404 page explaining why they've ended up there, give them suggestions for top pages and a search function to find what they want. Bounce rate will still probably be quite high.
You can request removal of URLs through WMT or also tell them about a change of domain, but I'm a little unclear as to what you've actually done.