You can have Orbeon Forms generate a schema for a form you created in Form
Builder, but the structure of the XML document used to capture data is
strictly determined by the structure of your form in Form Builder. You can't
have a different structure, and "map" fields to elements in the XML
document. If you need a different structure, you'll want to do the mapping
from the XML document produced by Orbeon Forms to the XML document you need
in a service Orbeon Forms can call when users submit the form, this for
instance with XSLT.
And you'll let me know if you were looking for something different, or I
misunderstand your question.