Hi Vedha,
You can implement custom XForms code that takes the content of textarea, and replaces an instance with the parsed content of the text area. We have an example that shows how to do this with an upload, instead of textarea, but hopefully this will give an idea:
http://wiki.orbeon.com/forms/how-to/view/upload-and-download-instanceAnd of course, be aware that in some cases, this isn't desirable from a security standpoint, as this allows users to bypass validation rules, change part of the instance they normally don't have access to, or even change the structure of the instance in ways that can can cause problems to other software using that data.
Alex
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