Flash Pro CS6.
symbolItem.exportToLibrary(frameNumber, bitmapName)
frameNumber An integer indicating the frame within the symbol to be exported.
bitmapName A string indicating the name of the new bitmap to be added to the Library.
Nothing.
Method; exports a frame from the selected instance of movie clip, graphic, or button symbol on the Stage to a bitmap in Library.
The following example exports the first frame of the currently selected symbol instance to a new bitmap in the library that will be called "mytestBitmap":
fl.getDocumentDOM().library.item[0].exportToLibrary(1, "mytestBitmap");