I assume you got all accounts in one folder:
like
X:\accounts\acc1
X:\accounts\acc2
and you want to create subfolder
X:\accounts\acc1\2016
...
In this case use this command in command line (you can open command line when you type cmd into search box on windows)
for /D %G in (X:\accounts\*) DO mkdir %G\2016
replace X:\accounts for path to your accounts folder. This command will list all subfolders in specific folder (X:\accounts) and create in each of those subfolders another subfolder named 2016
You can try it on some test folder to see if it do what you want.