Have you looked at the documentation for using UI files using PyQt? It is pretty much the same thing aside from the difference in how you actually use the uic module. But once you have the designer file loaded and have a class type, the rest is the same:
This gives you the different variations of how you can use the designer file, either by subclassing it, or by composing it and accessing it through a ui namespace. You should then have access to all the components you had defined in your designer.
Come to think of it, if you were previously manually converting the ui files to python files, then everything should be exactly the same. You had said you already are able to load the ui file at runtime but you are stuck on accessing the components. Nothing should actually be different at this step, if I understand you correctly.
So, regardless of whether you are manually converting a ui to py, or if you are loading it dynamically at runtime, once you have it as a loaded class type, you should be able to access your listwidget. Populating a listwidget is no different regardless of whether it came from a ui designer file or one that you created purely in code. You just access the listwidget and call its methods to add item.
At this stage, I think you are going to need to provide a minimal example of where you are really stuck. Because right now what we are doing it going over a large amount of information: converting ui files, loading ui files, subclassing/composing the loaded ui class, adding items to a list widget.
If you can manage to really ask specific questions, with examples, I can guarantee you will get more specific and useful answers. But I don't think I want to continue stabbing in the dark with answers that may or may not cover what you are really trying to do.