Serial number or process instance number

I need to do some XML field population and manipulation when the submit button is clicked on a plan page. How do I get the serial number or current process instance to access the XML data to manipulate it. Or what should I do to access that XML data. I want to basically do what the guy in the following topic does but for the current process instance on the plan page
http://forum.k2workflow.com/viewtopic.php?t=1084

So in short I want to populate some data in XML fields when I click the submit button. And no I cannot create a textbox and link the fields to the xmlfields because I would need thousands of text boxes to do it that way.