I'm assuming you mean a hyrdation system. My camelback will only freeze in the hose/bite valve. There is a pocket in the hose holder/shoulder strap for a non-toxic one-time use hand warmer. I haven't used it as I simply blow back (or hold the bite valve high enough) so the water isn't in the hose.
Once you start introducing things other than water into your reservoir, you need to really maintain it or you get the creeping black crud. With just water a rinse with a weak bleach solution generally keeps the black at bay, but with anything but water you also have to make sure you get rid of any of the in solution or suspended food stuff. Very hard to do.
If not a hydration pack, then consider that ice forms at the top, so store your bottles upside down so the ice is at the bottom of the bottle and the opening is ice free.