Larry,
Think of it like your flash light... you put the batteries in positive to negitive to increase voltage this is called series.
Now to keep the same voltage but increase amps hook the batteries positive to positive and negetive to negitive this is perallel.
But since you want to add batteries to your 24 volt system, you need to add two 12 volt batteries. like this:
your current batteries are in series to make the 24 volts, so you do the same thing with the other two and treat each pair like two individual 24 volt batteries.
Now hook your two 24 volt batteries in parellel.
If all of this makes sense it should go like this:12volt A to 12volt B == positive A to Negitive B == 24volt AA
12volt C to 12volt D == Positive C to Negitive D == 24volt BB
24volt AA to 24volt BB == Empty Negitive AA to Negitve BB and to negitive side of trolling motor
Empty Positive AA to Positive BB and to Positive side of trolling motor
And to charge you would use a 4 bank 12 volt charger
or a 2 bank 24volt charger
Note:You would NOT want to add one battery to this because:
If you hooked it up in parellel it would try and charge the 3rd battery to 24volts.
Bad idea for the 12 volt Battery If you hooked the 3rd battery in series it would then be a 36 volt system.
Again Bad Idea for a 24 volt motor I hope this helps
Jim