Is there a way to upload a document as a template so we can use @@Employee@@ tag to request a signature?

Currently I have developed a Nodejs application that uses data from a DB to create a PDF that later gets uploaded to Personio using the API. I'd like to add a signature when the document is signed using the @@Employee@@ tag but it is only usable manually creating a new document as an existing template which has to be a word document.

Is there a way to add the tag to a file generated through code? Whether it is a PDF or a Word document?