You can't expand the internal ROM memory, but you can add new sound with Expansion Packs.
Your Genos has, after the latest update, almost 4 GB of flash RAM memory for new sounds from Expansion Packs.
I personally don't know if any 3rd party has any expansion packs with acoustic pianos, as far as I know Yamaha doesn't have any for sale.
So do a quick search and maybe you can find what are you looking for.