This addon can export and import relative shapekeys in the scene.
for Blender 2.79 / 2.80
Download blender-shapekey-exporter.py and install it.
Open Tool Shelf by Ctrl+T (2.79) or Sidebar by Ctrl+N (2.80) and you can see the "Shapekey Exporter" tab.
This button exports shapekeys to save file.
This button imports shapekeys from save file.
Caution: All shapekeys that defined in save file are overwrited! If you want partial import, edit save file and delete unneeded entries.