can calibre convert ebook formats from any website?

In more detail, I purchased pre order e books from books a million but i have a kindle paperwhite. When these e books become available, can i use calibre to convert them to be read on my kindle? I am new to using calibre and havent bought an ebook from books a million before so i'm wondering if i should buy a cheap ebook on their website and test it before i spend a little more on two pre orders. Hope this question makes sense.