First, I’m assuming you’re already using one datafile to get a basic data model and this is a second data file you’re working with.
There are probably better ways to do this. However, you can pull in the contents of that text file by using “loadhtml”
As an example:
var path = "file:///C:/temp/test.txt";
var text = loadhtml(path);
text = text + " MORE STUFF";
This loads the contents of a text file located at c: emp est.txt, adds the words " MORE STUFF" to the very end of it, and displays it on the page. On my test document I got a cheerful little message of “test text MORE STUFF” displayed on my page when I ran this as a selector script.