Q: Can I add older pictures to my photo stream?

There are a couple of ways to do this:

1. Open the photos one at a time and take a screenshot.  This will add a new low resolution copy of the photo to the camera roll, which will then upload to your photo stream;
2. Email the photos a few at a time to yourself, then save the attached photos to your camera roll.  This will add a copy of the photo at full resolution to your camera roll (if you choose to send them as Actual Size) and upload to photo stream;
3. Use an app like PhotoSync, which will allow you to wirelessly transfer copies of all your old photos to your computer all at once.  Then delete these photos from your camera roll and use PhotoSync to transfer them back from your computer to your camera roll.  These will be the original resolution and will upload to your photo stream.  (You need an app like this because iTunes will not transfer photos back to your camera roll, where they must be in order to add to your photo stream.)

Mac users can use iPhoto to add and remove photos from My Photo Stream which will sync to other devices adding and removing them from the devices. Not sure if this functionality is in the Windows iCloud Control Panel.