If I understand correctly...
Place two T's in the pure water output line. Run a line off to the holding tank and another to a valve that can be opened manually to fill buckets/etc. for water changes and water for plants and etc.

When the float valve and manual valves are closed the water will get pushed into and fill up the pressure tank. When either valve opens the pressure tank will push water out.
Is there an automatic shut-off valve on the unit? When the output pressure reaches around 12 psi the auto shut-off valve shuts down the water at the input source. That's important for extending the life of the float valve.
Aloha