Normally, the voltage coming into the controller from the solar array is of a higher voltage than what the controller feeds into the battery. That's how MPPT works.
Could you go into more detail as to why exactly you see the need to charge at a higher voltage? Is your wiring getting hot, or are you trying to increase capacity? Is it because of the amp limit of your controller? I can see that your battery capacity at 100Ah is rather small. As an alternative to implementing just one more battery to make 36V, buy one more, and do either 2S2P at 24V or 4S1P at 48V.
How much solar do you already have? With that 24V Renogy controller, you can go up to 40A, so at 25V charging that works out to be 40A X 25Vcharging X 1.175fudgefactor = 1175W of panels. Call that four 300W panels. Realisticly, you need about 750W to properly keep that 2S2P sized battery charged. (200Ah/8) X 25Vcharging X 1.175FF = 735W.
