There is no way to select two or more adjacent notes of
different pitches at different positions and merge them together
into a chord of the same notes.
The user should be able to click a "Convert to Chord" button on
one of the toolbars which would convert the current selection of
multiple notes (and rests) into a chord. There could also be
popup menu options for choosing how to deal with the
conversion of notes and any rests in the current selection.
For example, the position of the new chord after the conversion
could be the same as the original position of either the first note
or the last note.
Any rests in the selection could be either deleted or added to the
duration of the longest note in the chord such that the total
duration of notes plus rests is left unchanged by chord
conversion. Each of these choices would be on a popup menu
for the "Convert to Chord" button.