grafting hybrid apples

If I get a pip of an apple to grow and then graft the result onto an existing apple tree can I expect fruit from the grafted twig?

Doug says I don't see why not. If you're asking can you start a seed and then graft the resulting small started plant onto a mother plant - yes.

Timing is everything but it can be done. Simply understand that the variety will determine the success as will your location. In other words, if you're trying to graft a warm-season apple (Granny Smith) onto a cold season tree (in Canada) then the Granny Smith isn't hardy and won't survive to produce. Get the right fruit on the right tree in the right climate and you can do this.