I have a Nook color that I turned into an android tablet. I can plug it into my computer and find the file where the Kindle books go, and just paste the .mobi file into that folder and then I can go to the On Device tab and find the book. I also have an android phone and do the same with that. And the book actually syncs like other Kindle books, so if I read on one device I can find my place on the other. One thing I have noticed is sometimes the formatting isn't perfect. Like there might be a space where there wasn't supposed to be a paragraph break or there is a hyphen on a word that isn't split between two lines. But for what I pay for the books, I can live with that.
Can you plug a Kindle Fire into your computer and get access to the file system on it? If you can do that, you should be able to find where the books go and add to that folder. I just bought all three Fifty Shades books from there for just over $5.00 for all of them! I do wonder how they are able to offer the books for that price.