There isn't a simple way to grow an existing fossil partition. You could use the extra space on the SD card for a second fossil partition, or a venti store. To rebuild the main fossil on the pi, you would need to boot from a root filesystem coming from somewhere else - a usb flash drive for example, or a networked server. Or you could rebuild the SD image offline, using another plan 9 or plan 9-like system.
It would be better to have a proper "installer" for plan 9 on ARM, not just a fixed image. Perhaps one day there will be one. If you are new to Plan 9, you will probably not find 2GB too limiting while you explore its possibilities. By the time you need more space, you will have learned enough that you will be able to think of lots of ways to expand.
