Software raid using the onboard raid controller or using ZFS? In case of ZFS you can do that using the webUI at "YourNode -> disks -> ZFS -> Create: ZFS".
Also make sure to wipe the disks using the webUI first, in case it came with a filesystem by the manufacturer. Otherwise PVE might not let you choose them to create the ZFS mirror.
But depending on what you want to do with it, it might be better to make a raid10 out of your existing raid1.