Maya help - search for shelf info - very handy info
(Add a tool, action, or Maya script to a shelf)
copy = control + shift + middle mouse button and drag to required area
Copying wont pass on the double click action. Looking in the My Documents\maya\<version>\prefs\shelves section and opening one of the files shows that a -doubleClickCommand is assigned to control double clicks. I dont think editing the file works as maya refreshes it on startup (I havent confirmed that tho)
Alternatively have a look into the Commands or CommandsPython sections of the manual and it shows you the script editor commands to create shelf buttons (shelfButton) and control all the elements including double click.