The existing root ball needs the plant above the ground to be able to sustain its health - its a two way thing - roots support the plant and the plant makes the energy for the roots to grow and exist.
Grafting a seedling onto an older and much larger rootstock would mean most of the roots dying, if not all. due to the small seedling being unable to produce enough "goodness" and energy to keep the roots healthy. Sounds tempting, but as with most things in life, there are no free lunches!
Hope this helps,........
Organoman.