I want to buy a 2 bay NAS. I want to install a 1 TB HD as a single volume, later I want to add another 2TB HD with data inside it, ext4 file system. Can I add the 2TB hd to the 2 bay NAS without data loss on both HD?
No, NAS using ext format and its own designed partition.
You may try to connect it via USB, copy the file out and then insert it.
Format and clear all the data inside.
Finally, copy the data back to your HDD
After you have created the RAID, you can move the whole RAID set out and put in into another NAS from the same brand, like Synology, without destroying the data. However during initial RAID setup all HDDs will be formatted.