Lets say that I have a MovieClip with the name of "Face" inside the library. I also have a different MovieClip in the same library called "Eyes".
How can I make an instance of "Eyes" and add it to the "Face" MovieClip?
var mainFla = fl.documents[0];
var mainFlaTimeline = mainFla.getTimeline();
var mainFlaLibrary = mainFla.library;
Another way to say this is, imagine this is Flash. I would double click the Face library Item. Drag a copy of Eyes to the Face timeline and that's the job done. Face now contains an instance of Eyes.
timeline and layer and frame appear to have no "addItem" method as I may expect and all examples appear to assume I am adding an instance to the Stage of the document.