To add more swap on a ZFS system:

# zfs create -V 64G -o org.freebsd:swap=on -o checksum=off -o compression=off -o dedup=off -o sync=disabled -o primarycache=none tank/swap2

To add it:

# swapon /dev/zvol/tank/swap2

To resize you could first remove it:

# swapoff /dev/zvol/tank/swap

Then destroy it:

# zfs destroy tank/swap