if you have existing physical aspx files, you can add them to the sitefinity sitemap by linking them as "external pages". this puts them in the site navigation tree (and I beliefve it will even index them for the search engine) as read-only elements. You would use traditional editors like visual studio or notepad to edit them.
if you want tighter integration into sitefinity, you can wrap the main logic for each page into a user control, which can be dropped onto a sitefinity page.
I've done this with an existing application for our website. we had a set of aspx pages for handling billing, which was protected by membership sign in. I simply added the membership provider to the sitefinity web.config, and converted all of the pages into user controls, making sure that the login controls all used my additional membership provider.
it's only slightly tricky, but if you know how to use standard asp.net practices it's not difficult.
also, yes you are allowed to use telerik controls within your sitefinity site, they are fully licensed and included with the package.
I hopet his was helpful! if you need further clarification, let me know and I will do my best to help!