There isn't a way to do this I'm afraid.
Photo Stream only adds photos from your Camera Roll, so other than taking a screenshot of an existing photo there isn't a way.
Remember though, Photo Stream only keeps photos for 30 days or up to 1000 photos.
So after a while, whatever you wanted to manually go there, would disappear again.
Think of Photo Stream as a backup until you get to a computer to then arrange your photos into albums and folders.