I'm surprised that no one else has chimed in with the answer. Unfortunately, I am not at my own computer to verify the procedure I will post in this thread but the source is reliable. Expressions are basically Face morphs which should be applied to any character of a base figure without altering the character morph. The way to save an expression, select the Add To Library icon at the bottom of the window which will launch the New Set dialog box. Click the Select Subset button to open the Select Objects window. The Select Objects window will list all the morphs in the Face and they will all automatically be selected. Deselect all the morphs that you do not want to be saved then click OK to return to the Select Objects window and OK again to save expression into the Face Library. Additional dialogs will pertain to single frame pose or animation poses but you determine the appropriate category.

Now if the character face morph comprises of several morph dials, I would recommend spawning a new morph dial which will compile all the active morphs together before applying any expressions morphs.