How can I get those books into my amazon account without them charging me through the nose for it?
If I'm understanding what you're asking, the answer is, you can't.
Folks have already given a variety of suggestions as to how to load content not from Amazon on to your Kindle. BUT, that content will not be part of your "Amazon account". Amazon will back up purchases you make there, and there are tools at Amazon to note when you've read books and how you liked them and all. But a book purchased from elsewhere than Amazon is not stored there.
If you want to not risk losing access, your best option is to back them up yourself somehow on your own system. Actually, there are a lot of people who back up their Amazon purchases as well. . .just be sure to keep track of which Kindle you're backing up because you can't load books from one Kindle directly onto another.
As to the 'I have no ATT access' question. . . . .it is fairly straightforward to load books you download via your computer. NO third party software is needed, though many here find Calibre to be a useful program for managing their ebook library. There is a PDF file of the Kindle Users guide that you can download and gives you step-by-step instructions for "side-loading". Look under "Kindle Support" which is probably a link all the way at the right near the top of the Kindle product page.
Good luck!