Iansoh is right - the bass buttons (played by the left hand) has standard chord. There' a button for each chord of the scale in major, minor, 7th and dim.
I agree with iansoh re: the use of a controller. I believe the best one to use is the expression pedal since both hands will be playing and you have to control the volume very frequently - the expression pedal therefore becomes the air pump (the bellow). Remember that the accordion is NOT velocity senstive and the volume of the notes are controlled by the the bellow. However, I believe many accordion sounds from hardware keyboards are velocity senstive. This will create problems since you'll get different loudness with each note - not possible with the accordion of which the volume is the same for all notes and the bass.
On the real accordion, if you play less notes on your right hand, there will be less air-leakage and hence you don't have to move the bellow much to produce a loud sound. If you press more notes simultaneously, air-leakage will increase and hence left hand must push more to get more air in to produce the same volume. I guess this may or may not help during sequencing, but important to think accordion while sequencing accordion!
So I would suggest programming the notes at a fix velocity, say 100 or 110, then assign the expression pedal for volume control.
Best way to learn - try to play it yourself! And listen to accordion playing.
Here are some accordion mp3 done with samples - not real accordion, to give you an idea. (accordion from Bardstown Audio)
http://www.bardstownaudio.com/frames/mp3.html (look for those with accordion)