You should not have to wipe your data. A lot of NAS support RAID migration or what they sometime call dynamic RAID (I have seen xRAID or flexRAID used). These allow you start with 1 disk (no RAID), the 2 disks (RAID1), then 3 disks (RAID5), etc. Check with your NAS manufacture to see what they support.
If your NAS does not support RAID migration, then this is still possible if it supports creating multiple RAID volumes (ie, have two or more separate RAID setup).