How to add a disk with existing data?

[SkyDiver]

Hi, got my AS6204T today.
My aim was to store my existing HDDs from my old PC into the NAS, however, the initialization process wipes out all data off of any disk that it is fed.
I have a 128GB SSD, so I created an image of it and used it for the initialization process.
Now I'd like to add my other HDDs, but I'm worried that they'll get wiped out as well.
How could I add my disks to the NAS without wiping them out?
Thanks.

[orion]

You can put your disk to USB box, then plug into NAS USB port (as an external HDD). I cannot find the other ways.

[SkyDiver]

So it seems... I already bought a box - this NAS. Now I'll need to buy another... weird...