You should not have to use the Melody line track from the midi you opened to populate the chords. importing the midi file should also import the Melody channel track along with all the other midi channels.

But, if I'm incorrect or misunderstanding what you're attempting, you can copy or move the Melody Channel from the file you opened and where all the midi channels are merged onto that single track. Right click on that track and select Track Actions| Copy\Move Tracks. From the Copy\Move Window, select the Channel that the Melody data is on and copy it to another destination track.

Conversely, to edit the Melody midi data, that can be done using either the Notation View or the Piano Roll View.

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