Because of the UrlRewriting Sitefinity uses, it would not be possible to use Sitefinity controls outside of Sitefinity pages. The recommended approach is to create Sitefinity templates (which are generated from masterpages, so you can just copy/paste your old templates) and create Sitefinity pages. The Sitefinity toolbox can be extended with user controls and custom controls, so you'll be able to use any legacy controls you have.
Finally, if you have some static pages that you do not wish to edit with Sitefinity, but would like to add them to the sitemap (navigation), you can create new external page and simply link to an old page.
I hope you'll find this information helpful. Let me know if we can help you with anything else.
the Telerik team