Alternatives: No, readpixel is only possible on pixmaps.
Possibility which I don't see as alternative but the RAM intensive way: Make own sprite objects (class) which holds the image and pixmap reference. That way you can read from the pixmap but draw the image. Whenever you update the pixmap, just use obj.thatImage = loadimage(obj.thatPixmap) (or similar) to update the image data.
Possibility which I don't see as alternative but the RAM intensive way: Make own sprite objects (class) which holds the image and pixmap reference. That way you can read from the pixmap but draw the image. Whenever you update the pixmap, just use obj.thatImage = loadimage(obj.thatPixmap) (or similar) to update the image data.