Nios 2 custom instruction.

Hello everyone, I am implementing edge detection on fpga using nios2 processor. I want to do custom instruction for sobel algorithm. Idea is that I will take pixel values (3*3 matrix at a time) as a input for custom instruction and it will give me final pixel value . But Question is that custom instruction provide only 2 inputs and i want to generate custom instruction which has 9 inputs. So can anyone guide me how to generate custom instruction in nios which has more than 2 inputs?

Thank you.