I just went through the process to import the kerning classes from one file to another but decided not to do so. On the window that lets me choose to import kerning values or classes I clicked “Cancel”, but Glyphs imported all the classes anyway.
While we’re on this subject, it would be useful to have an option to import all classes or only the classes for the selected glyphs. My reasoning behind this is that I wanted to import the classes for the small caps only, from a roman to an italic typeface. While the uppercase glyphs share very similar forms, the lowercase ones don’t.