Tom,
Welcome to the arranger keyboard forum and congrats on your PSR550. I am not aware that the PSR550 has a hold option, but you could, of course, add a pad sound to the style, and save it as a new style.
My Roland EM20 has this option in organ/split mode, but you can't run organ and arranger mode together. I wasn't aware that you could do this on the PSR550, until you pointed that out.
Just curious as to why you want this? Many styles have a pad playing during accompaniment anyways.
Starkeeper
P.S. I am a PSR550 owner as well as SqueakD on this forum. Marilyn is also purchasing a PSR550.