Yes, it's possible. You want to go into iPhoto's Photo Stream preference pane and set it up like this:
Now there will be no automatic upload to PS from your Mac.
When you want to assign a photo to a Photo Stream select the photos you want to add, click on the Share button at the bootom and select Photo Stream.
In the next window you can select an existing Photo Stream or create a new one
Thanks for your advice, but sahred photo streams still doesnt appear at the bottom?
Anyway, I've been trying to investigate ever since as I really would like to add to my same photostreams I am sharing and set up from my iphone... What I have found out is:
In icloud, it looks like I have photostream set up, ticked but I can see that its set up as my @me.com account? Really don't know why that is as I thought it was .com
Going back to my iphoto, I went into Preferences and then clicked on accounts and I see I have a @me and a @mac? so I thought I would remove @me to see if that helped? no idea if it would...
Anyway... I switched off photostream in iphoto and then twitched it back off, and nothing is showing for my photostreams.
I try sharing some photos to my photostream.... and once they upload, they don't show on my phone's shared photostreams.
Next thing I guess is..... logout of icloud of my @me account but it warned me about contacts which now really worries me? will I lose all my emails, contacts, diary syncing with my phone? help!........
Ask about the email in the iCloud community. I'm not sure how they are handled if you log out of your iCloud account. I have all my emails stored on my Mac so there's not problem. The other items, calendars, reminders, etc. will be removed from the Mac if logged out but will be added back when logged back in.