That's what I like about the Frontier Pro -- it is so versatile: direct screw onto any Platy bottle/bladder, inline use, or gravity feed.
In lieu of the AGG water bag, I use a 3L platy for my dirty-water resevoir (after punching two holes near the bottom, hammering in two grommets and stringing a length of cord for easy hanging). From there, it's just like with AGG: screw on the Frontier Pro and then let gravity draw the water through the filter, the hydration tube -- and into the receiving 'clean water' Platypus bladder(s).
One caution: the ultra compact and light weight Frontier Pro has a pretty big 3.0 micron pore size -- so it's only good for blocking protozoa cysts -- NOT bacteria and obviously not viruses either (if present). I treat my water with regular bleach, let stand for 30 minutes to kill bacteria and viruses-- then start the filtering to block protozoa, clarify water, and remove any bad water taste -- including chlorine.