How can I combine two filter kernels for image processing to create a single kernel?
Why not just do them one after the other?

Img = (Img * median) * sharpen;

I think you should look into how these filter operations actually work before attempting to use them. It's pretty simple, use wikipedia or even any image processing textbook. Even the Matlab docs have good explanations.

Median filtering is a non linear filtering operation which means that there is no fixed kernel function for it. You may define a neighborhood size but other than that, nope, no kernel. Have you looked into how it works? I dont know how you could generate a "kernel" for a non-linear filtering operation.

Maybe a basic refresher in Image Processing is in order? Perhaps even going over basics of Signal Processing / DSP? I would suggest looking over 2D filtering if you can.