Put them all in one fleet, and then tell each FHS to "Refuel Own Fleet" (Ship Overview -> Ship Design Display -> middle left dropdown).
This should cause the FHS ships to continually xfer fuel to the FHB ship.
However, since the FHB is not a tanker, it won't provide fuel to other fleets that target this fleet with "Refuel from..." orders.
So I think you will need to mark the FHB as a tanker.
If that causes the "Refuel Own Fleet" setting to stop working, you may just be stuck needing two fleets.
But not to fret. Give your FHB fleet cycling orders:
1) Refuel from stationary tankers (targeting FHS fleet).
2) Move to location (targeting FHS fleet).
With these orders, the FHB will continuously suck your FHS fleet dry, and you won't get any event interrupts.
EDIT:
If you use these cycling orders, that fleet will generate a lot of fleet history records over time--so many that it may cause performance issues.
To work around this, periodically move the ship to a new fleet with the same orders, then delete the old fleet.
This is one of the chores I have in my
annual checklist.