Yes this is possible, though best to back up before messing with partitions and filesystems
It looks like the partition hosts your root ZFS pool on your running system, so you should boot from your install media and select "Live CD" to change the partition size. Use the resize function of gpart(8). See the man page for syntax; something like gpart resize -i 3 /dev/da0.
For the ZFS pool to use the additional space you will need to set the autoexpand property of the pool to on. See the man page for zpool(8). This can be done from the Live CD when you import the ZFS pool (zpool import -R /mnt -o autoexpand=on zfspoolname) or your running system (zpool set autoexpand=on zfspoolname).
I was able to resize the /dev/da0 using gpart resize -i 3 /dev/da0, but autoexpand didn't work. zpool list returns zroot as the name. zpool set autoexpand=on zroot is accepted, but the available free space does not change.