Welcome to the forum. Interesting question.
I understand the concept, offline capability, and being able to upload and merge the data later.
There is not a way to submit back into an existing XML. You could possible have a "packaging" routine, that ran on the computer, that would perform the merging as a new form was added you could trigger a merge of two xml files.
Typically if you want to add more XML you would just create one form that would take multiple orders, etc and they would open it and save it each time. That would make the most sense.
Other methods I have seen is to run a local database that collects form data and then upload this when online. You may want to check out the Database Accelerator package, which is an installable set of web services to query and submit form data to SQL or SQL Express.
Qdabra Database Accelerator Suite Version 2.1 - Developer