Your storage pool is already configured with SHR (Synology Hybrid RAID) which means if you add a second disk then it will be added to this. The updated pool will still be SHR but will now have '1 disk redundancy' which is equivalent to RAID 1.
I'd say you're lucky because there have been quite a few people putting in a second drive to their default build NAS and wondering why they now have a mirrored RAID with the same storage size as before, whereas what they wanted was to double the storage and stay on non-resilient disks.
Follow the Synology help pages and use Storage Manager to do the upgrade. The data mirroring will happen as part of the upgrade.
Just make sure your new disk is the same as the current one.