I think it has to be in amazon format for the kindle to read it. The simplest thing to do is to use their conversion software. You can send it to amazon in word .doc format (using the kindle email account they gave you ie.
user@free.kindle.com) and they will email it back to you in their format and you simply need to load it on the kindle. The whole process takes maybe ten minutes.
If you are going to be doing this a lot and you don't mind spending a little money, I would recommend an alternate approach. In my view the speaking quality of the text-to-speech software in the Kindle is so bad, I have a difficult time forcing myself to listen to it. If you want something that sounds closer to a human reading it, I would suggest getting the software
http://www.nextup.com/TextAloud/ which is about $30.
Their default voices are also horrible, but you can purchase really good voices for it through their website. My preferred voice is 'Rachel' from Acapela, which costs $35. She has a modest British accent that I find very pleasant. So for about $65 you can have something that is much much better to listen to than the text-to-speech in the kindle.
I use this software to help proofread most everything I write that is longer than a few paragraphs. I find I can 'hear' things that are wrong much better than I can see them by merely reading the text. I usually have this software open on one side of my computer monitor and the master document in Word on the other side of my monitor when I am proofing what I have written.
I like to listen to audiobooks in the car and perhaps half of everything I listen to are things I have run through this software. I enjoy fanfiction and it is pretty easy to run it through this software which will output an MP3 file you can then burn to a CD for use in the car. You can convert pretty much anything you can get in .txt or .doc format.
The kindle is good for reading books, but I think there are better solutions for text-to-speech.
Duane