You can create a “Junction” to any folder on your computer inside your “Box Sync” folder, and then all of your files in that folder will be automatically synchronized to box.
cd "\Users\some_username\Box Sync"
mklink /J "My Documents" "c:\users\some_username\Documents"
dir "My Documents"
This is from This post on the Box forum
If you need to remove this junction later, use e.g.
rmdir "\Users\some_username\Box Sync\My Documents"