Hello,
You could call mergeDocx as many times as needed if you need to merge some DOCX keeping the sections and other DOCX being merged as continuous. Premium licenses allow to do all these steps using in-memory, so you don't need to create a merged DOCX file for each step (please check the included samples in your package).
You can use the previous approach or DOCXPath, there's no other solution to accomplish what you need.
Regards.