Can one stream photos to Shared Photostream from Finder on a Mac?
You can use iPhoto to add to a Photostream from your Mac.

I have Aperture using mine so I cannot fully test it with iPhoto without switching it off in Aperture. But is should work the same way by importing the images into iPhoto which also allows you to organise them. 

And then using the Share feature to share to iCloud. ( photostream )

The share with iCloud startup dialogue in iPhoto:
 



You can have multiple iPhoto libraries and open any of them as the Library to use by holding down the option/alt   ⌥  key when Launching iPhoto.

You should get a dialogue similar to this:



This means you should be able to import a SD card to it's own Library
Although it maybe easier to just organise within iPhoto and a singe Library



When selecting an image in Finder. The Share contextual menu does not offer up iCloud. So it looks like no with Finder at the moment