I think this is how Band in a Box works. 

PeteL
Didn't Craig develop a (free) chord library that you just drag in a MIDI chord by name from the browser? Perhaps it's still available.

It was audio guitar chords, not MIDI.

If you want to create chord progression (and from it arps, bass lines, melodies) by mouse, with all love to Sonar you need a DAW which support that. Sonar does not (at least not at the moment).
MIDI VSTs, CAL scripts, pre-made MIDI, Sonar arpeggiator are good in case you need that once or rarely, or in case you want play chords by one-three fingers (f.e. Yamaha style accompaniment). It is simply to much work when you want convert Amin "i iv V7" / "Am Dm E7" progression into music. Known to me methods:
* One Man Band . Can convert chord progressing into complete MIDI arrangement using sty styles.
* Band In A Box . Power modern tool in the same direction but with more possibilities (and corresponding price)
* Tracktion Waveform. A DAW with chord progression support.

With Melodyne Editor and careful playing you could play the notes on guitar and then convert them to midi.

You could use step record and the midi keyboard to get the chords into a midi track.

I haven't seen anyone mention step record here.

There is an on-screen touch keyboard or you can use a regular qwerty keyboard to play into the step recorder.

I did not mention it because it was not clear the OP could form the chords on a MIDI input device. It would be my preferred MO over CAL. If a guitar player, MIDI Guitar 2 would be an option too. Faster than converting audio to MIDI with Melodyne.

Anderton

PeteL
Didn't Craig develop a (free) chord library that you just drag in a MIDI chord by name from the browser? Perhaps it's still available.

It was audio guitar chords, not MIDI.

I was just about to mention that. Great library. All audio

Both the CAL scripts and the MidiChords require that the root note be entered then the chord form of the root be chosen. The on-screen keyboard would be good for this.

Are you not getting on with InsertPiz Midi Chords Dan?