Well I think you can achieve what you're after, but I'm not 100% clear on what that is.
The first thing is that you must have the same page extension for the whole site. You can use .htm, .aspx or even .asp if you want, but you can't have half the pages with .asp and the other half with .htm. You can, however, make your old Urls work by putting a redirect on them, so oldpage.htm will be forwarded to oldpage.aspx (or whichever extension you choose).
The first thing you must do is map the IIS configuration so that the ISAPI handler for each file extension points to the aspnet_Isapi.dll in the microsoft .net directory. This is done through the IIS manager.
After you have done this you should choose the page extension in the Url Master settings. This will be the page extension you want your site to run with.
You can then set up your pages in your site, and create all your site content. Once this has been done, you need to create the redirects or Url mappings. For example, you have dataaccessplugins.htm, and you want to forward all requests for that old url to the new one at data-access-plugins.aspx. Select the 'data access plugins' page in the 'Admin->Page Urls' list. Then, add a new Url for the page, and enter dataaccessplugins.htm, and set the action to '301'. Save the changes.
Now, whenever someone requests dataaccessplugins.htm, they will be forwarded automatically to your new DNN page.
I would also take the time to rename the pages to something a bit more keyword-rich and friendly. You don't need to stick to the same url of 'dataaccessplugins.htm'. You can create a new Url for the page of somethign like 'data-access-plugins-information', or whatever you like. You do this on the same Page Urls page, by adding a new Url in, and setting the status to '200'.
The best thing to do is experiment with it and understand how it fits together.
-Bruce