I am not sure if I understand your question correctly. Based on my understanding, you want to expand the external memory of CYW43907. The easiest option is to change the SPI flash to a bigger size SPI flash, size of which is limited to 64 MB. The list of supported flash can be found at 43xxx_Wi-Fi/WICED/platform/MCU/BCM4390x/peripherals/spi_flash/spi_flash.h
Please update the thread if you are looking for help in a different direction which I did not really understand.