Duplicate your melody track and open the copied part(s) in the Key Editor. Select all the note events and alt-drag them up and down to duplicate the line several times to get the number of notes you’d like in your final chords. Don’t worry about the pitches of these transposed melody lines. But do move the lines so they are roughly in the range you want the final chords. Then in that track’s Inspector on the Chord Tab turn on “Follow Chord Track.” You’ll probably want to mute the Chord Track so you only hear one set of chords playing.
Also if you turn on Live Transform in the Chord Tab you can just slap away at a keyboard and it will turn them into proper chords - which is great fun for folks like me with crappy keyboard skills.