You get a reference to the list of strings from the list of list of strings (eg by calling it's get() method) and then call it's add() method passing in the String you want to add.
Do you have a list of list of strings in the code shown?
BTW line 6 is overly complicated by you trying to do way too much on one line. I suggest you break into at least 3 lines, it will make your code easier to read and less prone to bugs.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com