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Let's say I have Garageband for the iPad (or other Synths and Apps) and want to play them through my Digital Stage Piano which has a USB Host Adapter. (meaning it behaves like a SoundCard and connected to a MacBook it works out of the box).
What Kind of Cable/Adapter do I need?
If I'm understanding correctly, you want to use your keyboard as a midi controller.
Here is one example of an iPad midi interface that should do the trick:
http://www.amazon.com/gp/aw/d/B006L9FTMI/ref=mp_s_a_1_1?qid=1417364974&sr=8-1&pi=AC_SY200_QL40