How can I add one disk to an existing raidz zpool?

You can't expand an existing raidz vdev, you have to blow it away and create it again with the new drive(s). See the other answer for better details.

Side note: Someone actually worked out that it's technically possibly to add drives to a raidz, but the functionality hasn't been implemented. The same is true of removing a disk.

See this thread from one of the FreeBSD lists: http://lists.freebsd.org/pipermail/freebsd-fs/2009-June/006327.html

To quote Dan Naumov,

To reiterate, you cant just add a single disk drive to a raidz1 or raidz2 pool. This is a known limitation (you can check with SUN ZFS docs). If you have an existing raidz and you MUST increase that particular pool's storage capabilities, you have 3 options:

1) Add a raidz of the same configuration to the pool (think 3 disk raidz + 3 disk raidz or 5 + 5, for example)

2) Replace each (and every) disk in your raidz pool one by one, letting it resilver after inserting each upgraded disk

3) Backup your data, destroy your pool and create a new raidz pool with a bigger amount of disks.