Option 1- put AF in bucket & pump it into tank using drill-motor pump available @ Ace hardware, Home Depot, etc.
Option 2- hook hose to MH only, @ open end w/funnel, fill hose w/AF, hook to city water & run AF into tank pushed by city water
Option 3 (if you have canister type filters)- remove filter media, fill canisters w/AF & reinstall caisters; hook up hose & fill tank w/AF-water mix.
#2 & #3 might require multiple doses to get enough AF in tank.
Another suggestion, if this is to be an annual ritual, is to cut a bypass into the line between tank & pump, like your previous example, then do #1 using the coach pump & bypass line. 1/2" OD "push" type fittings by John Guest
make it childs play to do. These are the same type supplied w/the Vagabond RO system kits. Add valve & tee to the tank/pump line, w/a valve on the tee leg, then as much tubing as you'll need to get to your bucket.