I understand your questions, but I don't see why you would want to do that. In DNN, you have user roles that have different access to different things. The stuff you mentioned (user accounts, creating portals, editing pages) is restricted to users with admin (or host) privileges, but the rest can be accessed by whatever roles you determine to have edit rights. You can set those for each sub folder of your portal (under /portals/{PortalID}/), for each page, and for each module within each page.
You even have the option of securing folders so their content can't be accessed simply by entering the URL into the address bar of your browser, but only via the DNN site (and only if you belong to a security role that has permissions on that folder).
So it should be possible to import EVERYTHING directly into DNN, including all your user accounts (if you don't know the passwords of those accounts, have DNN generate random passwords and send out notification emails to the users with the new URL, their login name and the password). Re-create your pages using DNN and any of the standard or commercial modules, and copy your protected files into a protected folder within the DNN framework.
A completely different approach (but one that to me doesn't make much sense) would be to embed your existing site through an IFrame. But unless you need any of the DNN functionality, I wonder why you'd want to migrate in the first place if you continue using the old application... You could also create a DNN module that does exactly what your old application did, using the old DB. Again, I'm not sure this makes too much sense though...
Hope my reply is making sense. If not I'll be happy to try and clarify.
Vera