Emailing and saving works, but this way is easier.
With a jailbroken iPhone with iFile on it, browse to the folder containing the photos (any folder). Tap on the blue arrow to the right of the screen for the photo or video wanting to be on the camera roll. This will show you the file properties. Scroll to the bottom of the file properties and tap on Add to Camera Roll. Voila!!
Likewise, if you tap on a photo from iFile as if to view it, there will be an icon at the bottom of the screen that looks like a folder with an arrow on it. Tapping that icon will add the photo to the camera roll.