...For that you need to place Dropbox directory on same drive as the where the SC2 bank files are created, then just make a junction:
Code:
c:\Users\user1\Dropbox\SaveGameBackup>mklink /J StarCraft2Banks "c:\Users\user1\Documents\StarCr
aft II\Accounts"
Junction created for StarCraft2Banks <<===>> c:\Users\user1\Documents\StarCraft II\Accounts
c:\Users\user1\Dropbox\SaveGameBackup>