I had a very nice "Japanese Plum" tree in my back yard until Katrina's flood waters killed it. They are fast growing and produce a ton of fruit that the local birds just love. Here in N.O. there are flocks of parrots and other exotic birds that have gotten loose over the years and have established a pretty huge population.
A neighbor had a JP tree in their yard that they never picked the fruit from. It was at least 4 time the size of mine and every evening that tree would draw the birds for an evening feeding. The noise those parrots made was incredible and visitors to my house would just sit in awe of the amount of them that flew in when that tree was in bloom.
I don't know if it will graft to the existing plum with success since I'm not at all versed in grafting, but it is worth a try to see what happens.