The base folder for file saving can be set at the 'Saving and Loading files' page.
I think you can set the 'Auto Move Older Screenshots' option ('Saving and Loading files' page) in a pattern like yours, but the files would only be moved there after (at least) 1 day. The pattern would be something like:
ss.%Y\ss%Y.%m_%b
I'm not sure if the \ could be part of that directory name, didn't try that, as I don't want to mess up my own archive.
On the 'File Naming' page you'd change the 'Default New File Name' to:
ss.%customdate%
and the 'Custom Date Format' to:
%Y%m%d_%H%M%S
And you really should change the 'Default Image File Format' to PNG (on the 'Image File Format' page), as that's way better suited for screenshots than JPG, as .png file compression is lossless while .jpg looses quality during the save. While that usually doesn't really matter for photos, it does matter for, relatively low-resolution, screenshots.