I know you can do the reverse (graft atemoya to cherimoya rootstock) very successfully. So, I'd bet that you could graft cherimoya to atemoya rootstock. Here in south florida, annona grafing is best around May/June when the trees are actively leafing out.

Jeff

I have only done a few cherimoya grafts, but I did them like I do for stonefruit. I collect budwood early in the spring just before the trees are pushing new growth. Then I store the wood in the fridge until the rootsocks are actively growing. I have pretty good success like this.