I do not think you can do that. if you want to merge Adaptive Form data with your existing xdp template you can do one of the two things
Create your Adaptive Form and base it on your XDP template(depending on the size of your xdp template this could cause performance issues)
Create XSD schema use it in your XDP and AF.
When the AF is submitted it is quite easy to merge the data with the XDP template
With Form Data Model the data you get on submission will be in JSON format and if you want to merge with your XDP template you will have to convert that into XML format