In theory, it looks like it is possible to email books (right click on the book and select send to [Kindle] email.) However, I tried it for the first time this morning and it doesn't seem to be working for me.
EDIT: Ahh...just had to add PDF format to the preference and use gmail. Thank you kovidgoyal (over in another thread). LINK:
http://manual.calibre-ebook.com/faq....ails-using-app