Here is a link to a good video on grafting a young mango root onto a larger branch of a mature tree.
https://m.youtube.com/watch?v=GV699_TBGIIThere are other videos out there showing grafting of multiple larger roots onto much larger trees on YouTube.
This simple technique may be useful for saving a tree with damage to its roots. It can also be useful for Bonsai or a slight modification to this technique may help with repair grafts.
Grafting Examples has some great videos out there.
Simon