From what I gather, the iPhone does have a host USB port (assuming you have the Camera Connection Kit to provide the connector), but it is not enabled in iOS.
iOS on the iPad is said to have USB keyboard support. I don't have the necessary hardware to back that up though.
There are bluetooth numberpads, though, those should work. Something like this: http://store.apple.com/us/product/H0321VC/A (just found it, never used)