Im not sure what it is you want to do. Are you trying to manipulate the power at the source? (receptacle). If so i would not recommend it, as part are manufactured a certain way for an intended purpose, so modifying the GFCI it self is not a good idea at all.
Now, from what i gather , your issue is that you are heaving problems already with the unit, in that its overflowing. Which means you limit switch for the pump, or the pump itself is not working correctly. Thus you want find an alternative at the power source that would automatically shut of the circuit if an overflow was present, correct?
if this is the correct understanding of what you want please reply and i will tell you my thoughts on that, otherwise i don't want to waste my time typing it