Attached Link: http://wiki.blender.org/index.php/Doc:Manual/Animation/Basic/Deformation/Shape_Keys

My understanding of how the manual describes "Relative" ShapeKeys is that they behave exactly like Poser's morphs in that they can be blended together in any desired combination.

You may or may not be aware of the existence of a program called MakeHuman -> http://sites.google.com/site/makehumandocs/what-is-makehuman