This post assumes that the zpool is using raw disks instead of partitions.
It is also assumed that the name of the existing zpool is pool0.

Start by booting the livecd and enter the shell.

Import the existing zpool:

# zpool import pool0

Create the new dataset for the root, and set the mountpoint to /mnt for the installation.

# zfs create pool0/ROOT
# zfs create -o mountpoint=/mnt -o canmount=noauto pool0/ROOT/FreeBSD
# zfs mount pool0/ROOT/FreeBSD

 
Fetch the installation files and install the distribution:

# cd /mnt
# fetch ftp://ftp.se.freebsd.org/pub/FreeBSD/releases/amd64/amd64/10.3-RELEASE/base.txz
# fetch ftp://ftp.se.freebsd.org/pub/FreeBSD/releases/amd64/amd64/10.3-RELEASE/kernel.txz
# fetch ftp://ftp.se.freebsd.org/pub/FreeBSD/releases/amd64/amd64/10.3-RELEASE/ports.txz
# fetch ftp://ftp.se.freebsd.org/pub/FreeBSD/releases/amd64/amd64/10.3-RELEASE/src.txz
# tar xvpf base.txz
# tar xvpf kernel.txz
# tar xvpf ports.txz
# tar xvpf src.txz
# cd /

 
Make the system bootable (change ada0 to your hdd):

# echo 'zfs_load="YES"' >> /mnt/boot/loader.conf
# echo 'zfs_enable="YES"' >> /mnt/etc/rc.conf
# zpool set bootfs=pool0/ROOT/FreeBSD pool0
# zfs unmount pool0/ROOT/FreeBSD
# zfs set mountpoint=/ pool0/ROOT/FreeBSD
# zpool export pool0
# sysctl kern.geom.debugflags=0x10
# dd if=/boot/zfsboot of=/dev/ada0 count=1
# dd if=/boot/zfsboot of=/dev/ada0 iseek=1 oseek=1024

 
Reboot and login as root, then change your password and do your usual configuration to /etc/rc.conf.