Mirrored vdevs are great for performance and it is quite straight-forward to add a mirrored vdev to an existing pool (presumably one with one or more similar vdevs already):
zpool add [poolname] mirror [device01] [device02] [device03]
If it’s a two-way mirror you will only have two devices in the above. An example for ZFS on Ubuntu with a pool named seleucus and two SSDs could look like:
zpool add seleucus mirror ata-SAMSUNG_SSD_830_Series_S0XYNEAC705640 ata-M4-CT128M4SSD2_000000001221090B7BF9
As always, it’s good practice to use the device name found in /dev/disk/by-id/ rather than the sda, sdb, sdc etc. names as the latter can change – the former do not.