I think our friend may be looking to add an automatic accompaniment, possibly preprogrammed to accompany him on his piano. In this case may I suggest something like a yamaha QY100? He can then program his chords and backing and play along on his piano? Of course a PSR2000 would also do very nicely, though it will cost a bit more than an accompaniment unit.
If he is asking if it possible to add the accompaniment to his piano physically, then I do not think this would be economical. It would be easier to go and buy a Clavinova!
If he wants to keep playing his piano and have accompaniment automatic in background, like midifile, then even pc computer with a sound module or software synth could fit the bill.
The question is not entirely clear, but given enough answers maybe we can answer it!
Regards
Simon
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Simon G.K. Williams
simon@svpworld.com
Creative Music & Multimedia
http://www.svpworld.com________________________