The option to make a tuplet from a group of selected
notes without changing the existing durations of the
notes doesn't work. If you select n notes and select
"Timing is already correct; update display only" in the
tuplet dialog, the dialog initially assumes you want
n+1 notes to be played in the time of n notes; if you
correct the values to be n and n-1, respectively, the
operation actually puts only n-1 notes into the
n-tuplet and omits the n-th note from the n-tuplet.
There seems to be no way to convert exactly n notes
into a tuplet which is played in the time of n-1 notes.