Assembler only works from PDF files. So before you call the invokeDDX service you'll need to convert you XDP into a PDF using something like RenderPDFForm.
Once that's done, you can use the invokeDDX operation from the Assembler service.
The way it works, you need to provide an input map of the PDFs you're going to use and a "document" variable that contains the XML representation of you ddx file.
The output of the operation is of type AssemblerResult, not OuputResult.
I was able to do this successfully. The issue that I am facing now that while merging 4 forms together.. if the 2nd or 3rd form happens to be in Landscape orientation, all rest of the forms also convert to the Landscape orientation. Also, when I try printing it, the Printer says the Printer Size did not match Paper type or something of that sort. Following was my DDX: