Not sure if you can do that. I had to buy a larger SD card for my TomTom to fit a French map on also there is a limit to the size of card you can fit. SD cards will work but some TomToms's will not work with SDHC (High Capacity) cards. The SD card must be a 2.0 GB standard card. You must download a map, an application and a voice on the card or you will get a red X. The unit will read either the SD card or the internal memory, not both.
Best Android apps 2015/2016: Best apps for your new phone or tablet - what apps should I download?