Canon 5D mark 1 comes with the Focusing Screen Ee-A. I like to have a custom grid on this screen. Is it possible to draw or maybe etch custom grid on this piece? If possible, how can we do it?
I would suggest rather than drawing directly on the ground glass focusing screen that you use a piece of clear laminate material to draw on. If you draw your desired lines onto paper, tape the plastic over the paper then trace your markings onto the laminate, you can try several different options depending on what you need. Once you find the perfect arrangement then you can mark it onto the focusing screen. B&H Photo sells Canon focusing screens for under $40 so it's not the end of the world if you mess one up, but this method allows you to try several different markings to find the one that works best for your needs.
I dont see any reason why not, you could easily do it with a very fine permanent marker and a ruler.