http://www.google.com.au/search?q=xhr+tutorial
The top five all look good.
Or were you after the server side part of dealing with an XHR request?
What is stopping you from using XUL UI in GAE?
So, to answer the question, yes you can send xmlhttprequest data to GAE.
But i suspect your real question is how. And for that, you are going
to get to learn a bunch of javascript and ajax. Poke around google for
xhr tutorials.