For pollinating purposes, is it possible to graft crab apple onto regular apple trees?
I've heard that is a common orchard practice and that crabapples are especially good pollen sources.
Apples are very easy to graft.
My "grafting mentor" has a nice crab grafted to the same tree that supports some dozens of varieties and it has done well for many years now.
Commercially, the Crabs are grafted onto dwarf rootstocks on separate trees. There can be some disease issues putting multiple grafts onto one tree, especially virus.
Once you graft a virus infected scion to the tree, it goes into the rest of the tree.
I would not graft crab apple onto standard for pollen. Plant it next to your regular apple on a dwarf rootstock.