Grafting onto espalier

I have some apple, pear and peach espalier. They are all about 2 years old with established cordons. I would like to increase the variety of fruit by grafting onto these trees. Any suggestions as to how I could do this?

Here is a reference that should be very helpful. I’m planning on espalier for my apples and I will be reading more about it too.

Based on what I’ve seen thus far, you want to have your first layer very well established before starting others so they get thick enough. That’s as far as I can help, many others are experts here.

I have seen images of some super long espaliers. You could extend it and graft to the end. Will need support of course. You could also make the ends go up a support like a U.
I had to let my espalier grow out as it was a primary target for deer.