If you are switching from a 'classic' asp site to a new DNN site, and you have search engine rankings you'd like to keep, there is only one option - that is to use 301 redirects. The 301 permanent redirect status is a notification to the program that made the request that the item requested has shifted permanently to a new location, and that it should update any links or bookmarks. In the case of your browser, this just means you see the page with the new Url automatically updated in the address bar. For a search engine bot, it's a notification to update the search index with the new address.
Assuming your site features well in, say, google, then if you set up 301 redirects from your old page to your new pages, you'll get a pretty fast update to the search index, and you won't lose any pagerank for your pages. WHen I updated url on my site, it took about a week, and the page rank didn't change. In fact my place in the search results started to go up slightly due to more friendly Urls with better search terms in them.
You can't issue 301's with the Standard DNN friendly Url provider. Even the site urls.config is just used to rewrite Urls, which is different to redirecting. Rewriting is just the process of changing how the Url looks (say, from pagename/tabid/66/default.aspx to default.aspx?tabId=66) as far as the web server is concerned - the requesting browser doesn't even know it has happened.
I have authored a Friendly Url Provider replacement that issues 301 redirects for old versions of DNN Urls, but even that won't help you out, as you are coming from a completely different Url scheme to the DNN scheme.
I have in Beta testing a Module called 'Url Master' which will allow you to do what you wish - 301 redirect the old Urls and point them at your new DNN pages. This module is probably going to be released early next week . With it, you can select a DNN page, then create any 'old' url you like which will redirect you to the chosen page.
Ie, new url: mysite.com/products.aspx
old url : mysite.com/products.asp
With this module you can define 'products.asp' as a Url with a 301 redirect to products.aspx, so that any visits or references to your old site Urls will automatically transfer the request over to your new site Urls.
You can read my blog entry about this product :New iFinity Url Master Module in Beta Test You can see screen shots and get a more in-depth explanation.
I'm happy to answer any questions about the whole process
It's OK from a SEO point of view to do short terms 302 redirects to the right location, however, long term you want to issue 301's to get rid of all traces of your old URL scheme from the search engine indexes. The quicker you do it the better, you don't want to get shuffled down the rankings.
You should also check with your host. Some hosting companies give you the ability to define a set of 301 redirects for your site, totally separate to your DNN installation. This would be the fastest way to solve your problem, and a method I have successfully used when I converted an 'old' site into a DNN install.
-Bruce