Not really sure Sengyan
You have a difficult time to this because you have one larger mother root feeding it! How far away is it from the trunk?
I was just thinking,.. leave it alone and graft on it, they can grow very close together, even a foot is fine!...Our trees don't grow huge anyway and you can always cut it down low. But...if you are thinking of moveing anyway, this is what I would do. Cut about 2/3 of the root with a saw the first year, [spring] the root leading to it from the main trunk, about a foot away from the sucker, also, cut the root going away from the sucker all the way.
Then in another year cut completely thru.
Konrad

Hi Konrad. My apologies for being this late in acknowledging your reply. Have been away. The sucker is too clost to the parent tree - 2 feet - to graft as you suggested. I am afraid cutting off the root might hurt the tree. I am thinking of air-layering it next spring. I hope it will have rooted by October.

Sengyan

Yes, you can air layer and see what happens,... not a sure thing.
Not sure if you understood my last post, said, at one foot away from the trunk would be O.K. to graft, you have 2 feet, that's plenty!
Some people put 3 trees into one hole when planting.
Konrad