The following pool is normal, not mirror, on one disk

18:02:00 root@eeny:~# zpool status
  pool: tm
 state: ONLINE
  scan: resilvered 3.11T in 16:27:30 with 0 errors on Fri Apr  1 13:07:34 2022
config:

	NAME        STATE     READ WRITE CKSUM
	tm          ONLINE       0     0     0
	  sdb1      ONLINE       0     0     0

I have another disk, same size and I would like to convert this pool into a mirror one

NOTE: For that, you need to use attach. DO NOT USE add because you will create a STRIPPED (RAID0) instead of Mirror (RAID1).

18:04:59 root@eeny:~# zpool attach -f tm /dev/sdb1 /dev/sdc1
18:05:38 root@eeny:~#

Now the pool is mirrored. Must wait for sync to be done

18:12:10 root@eeny:~# zpool status
  pool: tm
 state: ONLINE
status: One or more devices is currently being resilvered.  The pool will
	continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
  scan: resilver in progress since Sat Apr  2 18:05:32 2022
	242G scanned at 620M/s, 4.15G issued at 10.6M/s, 3.16T total
	4.13G resilvered, 0.13% done, 3 days 14:28:09 to go
config:

	NAME        STATE     READ WRITE CKSUM
	tm          ONLINE       0     0     0
	  mirror-0  ONLINE       0     0     0
	    sdb1    ONLINE       0     0     0
	    sdc1    ONLINE       0     0     0  (resilvering)

errors: No known data errors