Given that this question is a few months old, you may have already found an answer, but I figured I'd provide one in case anyone else comes to this post looking for an answer to the same question.
While I don't know of a way to add photos from the Camera Roll to the "main" photo stream retroactively, you can add pictures to shared photo streams. Those will sync up with iCloud and any iCloud-connected devices, just like the "main" photo stream.
To add pictures to a shared photo stream:
- Create a shared photo stream. On your iPhone, open the Photos app, go to the Shared tab (bottom center of the screen), and tap on "New Shared Stream". Give the new stream a name.
- Go to your Camera Roll, tap on Select (top right of the screen), and select (tap) each picture that you want to add to the new shared stream.
- Once all of the pictures you want to add to the stream are selected, tap the share button (bottom left of the screen), choose iCloud on the menu that pops up, and choose the newly created shared photo stream.