Yes, is is certainly feasible to graft onto an existing apple tree. I had four unpalatable cultivars which I originally obtained as scionwood from the Central Siberian Botanical Garden in Novosibirsk, back in the '90s. I consolidated them in one tree, since I did still want to keep them around, but without taking up so much space. I grafted a couple more on this tree last year, so now there are six, plus the original crabapple, on the one tree. I was feeding them to the
chickens, but have discovered that they're a great source of homemade pectin.