Hi Vedha,
The set of instances in a given XForms page is static; you can't add instances after it has been loaded. However, you can have a "stuff" as much data as you want in a given instance, and have it contain many "sub-documents".
I think this might be related to your other message (linked below), where you describe some issues you're having with a very large instance. Reiterating what I mentioned there, is there any chance you could create an example that reproduces the issue, so we can investigate it? (Also, since we now have a private communication channel in place, feel free to use it if it is more convenient for you.)
http://discuss.orbeon.com/Performance-Tuning-when-handling-huge-instance-td4661988.htmlAlex
--
Follow Orbeon on Twitter: @orbeon
Follow me on Twitter: @avernet