zpool attach
add to an existing vdev (ie. mirror/parallel)
zpool add
(always try with -n first) → add to the pool outside of a vdev (ie. serial)
zfs offline -f
dont. It was made to manually fault a device if ZFS didn't detect it as faulty yet, it is permanent. If you want to recover a pool where you did an errorneous zfs-add and then zfs offline -f like the idiot that I am you have to increase zfs_max_missing_tvds to one (zfs kernel-module parameter in linux) and you'll be able to import+mount it as read-only (which I then recovered using zfs send|zfs receive plus rsync for still getting the missing bits). Cannot import pool with one device forced offline #10254