Yes, it’s possible, but it’s risky because focusing screens are delicate and easy to damage. Based on the suggestions here, the safest approach is to first test your grid on a thin clear plastic or laminate sheet instead of marking the screen directly. Draw the grid on paper, tape the clear sheet over it, and trace the lines so you can preview different layouts.

If you decide to mark the focusing screen itself, use a very fine permanent marker and a straightedge. Avoid etching unless you’re prepared to ruin the screen, since scratching the surface can permanently affect visibility. Work carefully, use minimal pressure, and keep dust and fingerprints off the screen.

A practical option is to experiment on the clear overlay first, then only transfer the final design once you’re sure it’s useful. Replacement focusing screens are available, so mistakes are recoverable, but direct modification should still be treated as a last step.