I doubt that conversion of copyrighted ebooks will work due to DRM (Digital Rights Management) issues. It's how Amazon ensures you'll have to buy Kindles and only Kindles to read their books, and B&N does the same for Nooks and Sony, for Sony for Sony readers. They also want to ensure, of course, that once a book is downloaded, it can't be easily pirated.

For older books, conversion can probably be done, but you probably won't have success converting PDFs that are essentially image scans (as for most of the ebooks available from Google Books). That requires OCR (Optical Character Recognition), and the deficiencies of OCR are readily apparent when you read the EPUB (etc.) versions of Google Books titles. I do read a lot of EPUB books on my Nook, but the faulty OCR is quite annoying, and sometimes I have to double check bits against the original scanned images (NOT on my Nook) in order to decipher the gobbledygook.