"Sad that there is no documentation of this in the User Manual, the Reference Manual or the Tutorial."

The Poser library is fully documented in chapter 7 of the manual. Item 14 on page 94 states, "Save to Library (+) Click this button to add the item that is currently selected in your scene to the library". The annoted diagram on page 92 clearly shows where item 14 is located.

Hope this helps.