Hi Tivalli,
Sure, anything's possible with code. :) BTW, your XML is not well-formed - you don't have beginning tags for my:reg_code and my:reg_name.
InfoPath will automatically upgrade old forms and insert the new field. If you have <20-30 forms that might be the easiest solution.
There used to be a feature in SharePoint to allow demotion which would allow you to edit XML contents via a datagrid view but that option is gone.
We have a tool that supports created XML forms from an Excel spreadsheet. You could use that to batch add the value, but it would require generating the Excel and configuring it to map to XML. Could be an hour or two and you'll have to get the tool. We also have a DBXL migration tool (under downloads) that you can use to transform XMLs using XSLT. It will do a bulk transform and you don't need to buy anything but you'd have to install the DBXL Web service trial and then migration tool. You'd also have to right some XSLT. But, that would be an easier option if your XML is complex and you have thousands of XMLs.
Send us email if you'd like to pay for an hour of support.