Possible to graft?

I’ve got this Mexican fence post (Lophocereus marginatus) that my neighbors were throwing away, it’s in rough shape but healthy. It’s had this huge hole since I got it, and I’ve always wanted to try a graft, is it possible to graft something inside this hole? And how would I do such a thing? I’ve got plenty of cacti I can use, I was thinking of using Selenicereus grandiflora or maybe one of my little gymnos.

Is it possible?