I've always been kind of haphazard about expressions, but I'd like to be more systematic.  Mainly I'm interested in separating faces and expressions, but the trouble is that there's no distinct line between the two.

Is there a way to adjust head morphs (through the library) using addition and subtraction, rather than absolute values?  That way a figure could have for example a slight Mona Lisa smile, have a smile expression dialed in with a library preset, have it removed with the minus library preset, and still retain the "feature" of a slight smile afterwards.  This would allow me to use expressions more systematically.