No.
Most Kindle and iBooks titles are copy protected and tied to your account (and in the case of Kindle to your registered device(s)). On top of that they use different ebook standards, so that even if you have one of the few unprotected books you would first need to use a conversion program to change the file type.
Since most of the free/unprotected books are available in both stores, or through sites like the Gutenberg Project (multiple formats), it's rarely worth the trouble to convert them.