You can certainly add a new key, but I believe there is a maximum number of registered keys the ECU will recognise at one time. I don't know enough about the technical details to be specific on this.
As Fred says, the coding process is not overly complicated, basically just needs a wire with a resistor and follow the procedure, you can find it on youtube, as far as I know all Honda HISS equipped bikes use the same procedure.
You can get key blanks with the chip on the auction site for around £8, the chips are common type.