Is it possible to take an existing PDF that does not have any predefined form fields, alter it to a template PDF file to support field input and automatically fill up the PDF template with values from an XML file?
Yes It is possible in Adobe Livecycle. Since you have mentioned you are new, I am telling where to start. Adobe Livecycle ES is a collection of different tools that does a lot of functionalities. One such tool is Adobe Livecycle Designer. You can see many examples for how to create a form in this forum or using predefined template using the Designer tool's Help Menu.