Can I make existing libraries use external SRAM for the Arduino
Making a library use external SRAM is not an easy task. It can be cone, but it takes quite a bit of work.

The best start would be to abstract the SRAM into a library that provides simple access to it. Then you would need to use that new library to read and write data in the SRAM instead of accessing variables in the other libraries.

There is no way of simply mapping a variable into external SRAM - for that you need a microcontroller with an MMU  and/or an EIB.