In Sonar X3, is it possible to store a serie of chords
in the piano roll pane and, using an external MIDI
controller, step one chord at a time using a UP/DOWN switch
on the MIDI controller ?
Mostly only UP function could be used..
The purpose of this is that I'm jamming a lot with friends
and I'd use Sonar X3 as a 'standby' sequencer to play and 'keep'
each chords one at a time played while I play guitar at the same time.
The UP function would go to the start of a chord, play it and stay there
until another external UP function would come and step to the next chord available.
I think I would have to place every chord on specific time
intervals on the piano roll pane right ?
Thanks for your help.

Perhaps look into Matrix view. I don't use it but it might do what I think you want.

Check out the Chords module in Cthulhu (https://www.xferrecords.com/products/cthulhu). May be just what you're looking for.