The easiest way to do this is via IIS. 

First you have to find the path to the 404.htm error page (steps are for IIS 5 and 6)

1. Open IIS
2. Expand the nodes until you see your website
3. Right click the website and select Properties
4. Click the Custom Errors tab
5. Find the 404 error and click edit view its path
6. Copy the path as a backup
7. Using Windows Explorer navigate to the 404 error page
8. Copy the page
9. Rename it 404CustomRedirect.htm

Next you need to modify the 404CustomRedirect.htm page to support redirects

1. Open 404CustomRedirect.htm
2. Add the following meta tag to the site (be sure to change the URL to your site)
   <meta http-equiv="refresh" content="0;url=http://www.YourSite.com">

Next you need to update IIS to use this new page for the redirects

1. Open IIS
2. Expand the nodes until you see your website
3. Right click the website and select Properties
4. Click the Custom Errors tab
5. Find the 404 error and click Edit view its path
6. Change the file name to 404CustomRedirect.htm
7. Click OK and OK

Now try accessing one of the old sites URLs

NOTE: If you want to map the URLs so the old URL redirects to a specific new URL you can do this by creating a ASP.Net page for the redirect rather than a htm page and then do a switch/select case on the referrer URL and have it redirect to the page based on a matching referrer URL.

I hope this helps you.

Best Regards,

Robert J Collins | Co-Founder & President

Netlogic Corporation

 Unfortunately, I do not know of a built in DNN way of mapping URLs in this way. 

Best Regards,

Robert J Collins | Co-Founder & President

Netlogic Corporation