A quick question: Why?
The Kobo firmware can be updated by holding down "menu" and pressing power, keeping on holding "menu" until it displays a message saying it's "initializing USB firmware partition". At this point you can plug it in to a PC, copy the firmware to the FAT32 USB mass storage disk it exposes, then eject and unplug the Kobo. It'll reflash its self using the firmware file on the partition. This is the method used by Kobo Desktop to update the Kobo.
Why suffer through doing it with an SD card?