Technically, yes, you can add Sitefinity to your exisiting site. However, you will not be able to use Sitefinity functionality (in particular page editing etc.) with your existing pages. What you can do, however, is add your exisiting pages as "external pages" to Sitefinity, so that they appear in your sitemap, and for the new pages use Sitefinity to create them.
It is also worthy to note that when you have a web site with a large number of pages, you can use Sitefinity API to recreate these pages in Sitefinity. You can use this API and its methods to add pages and content dynamically to Sitefinity (for example if you have the data in some other database, or by screenscaping your existing website).
Let me know if I can assist you with more details on any of the mentioned topics.
the Telerik team