I don't think it is really possible to move things between different vendor stores. There might be one out there, but I have not seen it.
You might look into Calibre. It is a device agnostic e-book storage and organizing tool that can store all of your books on your computer. It can also convert the formats from one e-book type to another.
The MR Calibre forum is here:
https://www.mobileread.com/forums/fo...play.php?f=166