Sophia, it’s not hard to graft chestnuts, you can do it I don’t know much about Chinese varieties, as I’m in Canada and they are often not hardy enough where I live. I’ve grafted mostly american chestnut selections, nice and hardy in fluctuating temperatures like here.
I bet it’s the same though with the Chinese and hybrids. The easiest graft is a cleft graft; you take a twig with only last years wood on it, still dormant, and pop it in the fridge…in a plastic freezer bag. This keeps it dormant and moist. When it’s warm enought that your outdoor tree makes new growth, cut the end off a branch into the last year wood. Cut straight across and then make a slot with a knife in the cut end. take your twig out of the fridge, cut off a little bit with two buds and sharpen the cut end to a wedge. Slip the wedge in the slot you made and make sure the bark and cambium line up well on at least one side. Wrap it tightly with tape, electric tape from the hardware store works…parafilm is easier to use, as it rots after a bit and you don’t have to remove it later, but electric tape works too. If it’s hot out, you might ned a plastic bag over the graft to keep it moist while it knits up. Poke a few holes in the bag for air.
After the graft starts leaves, cut the corner out of the bag for more air…so the bag is not fogged up. Leaves turn black and rot if they touch the inside of a wet, fogged up bag. Maybe someone else will add to this too.
I do a lot of grafts into potted seedlings on the kitchen table, so not as much outdoors on sprouts and established trees. I bet castanea has some tips for you…perhaps he will see this thread and help you.