Mods like this actually require researching and trial and error.
Nothing you can buy for it.
I meant buy an analog stick that is 8-way (8 directions), with 16 pins (2 for each direction. I would then wire the up, down, left, and right to the d-pad, and the other four diagonal axis direction would each be wired to 2 of the D-Pads directions (Up-Left would press up and left on the D-Pad at the same time.