I know you said you want more storage, but I really wonder if the heat pump has the recovery capacity to make use of the extra storage tank.
Why not skip the circulator pump entirely? Run cold water from the street or your well into the cold water inlet of your GE tank, and set the GE tank to run in heat-pump-only mode.
Outlet from the GE tank will go into the cold water inlet of your second tank, and the second tank will be conventionally heated (electric or oil or whatever). Outlet of the second tank goes to your faucets.
That way the heat pump tempers the water going into the second tank, raising it from 50 degrees to whatever it is capable of. The second tank assures you have a "normal" quantity of hot water, but whatever energy the heat pump provides will reduce the energy needed by the second tank.
Of course, if the second tank is electric then as Freddy says, What's the point of the second tank, just use the GE in its hybrid mode.
Or maybe put TWO GE systems in series as above, the first in heat-pump-only mode and the second in hybrid mode; or both in heat-pump-only if that provides enough hot water. We have all kinds of ways for you to spend more money!