IF I understand what you're asking (very difficult when there is a total of one punctuation mark in your entire post, and your shift key appears to be broken)...
As you've probably figured out,
var usr = $("#username").val();
collects the value of one input field. Repeating that for additional fields should be straightforward.
The collected value(s) get put into the post on this line:
My guess would be that conventional urlencoding is being used, so
data: "username="+ usr+"&somethingelse="+anothervalue,
would put two values into the post. But that's just a guess; there is probably something more robust. Since you're using jQuery (which I haven't really used), I'd recommend reading its documentation on how to use the [font=monospace]ajax[/font] object and finding out.
Oh, and get your shift key fixed.