most likely your 320gb drives are striped (raid-0) into a single device. if so, you'd have to backup/restore the entire device since any changes will reinitialize (erase) the existing drives.
also, most raid-0 implementations require drives of the same size or they will only use the size of the smallest drive. so adding a 1tb drive would result in a 900+gb raid device with 600+gb inaccessible on the 1tb drive. intel's matrix raid would allow you to access the 'leftover' space.
if you're using JBOD, then you could add the 1tb drive but the question would be 'why'? it would offer no performance benefit and unless you've filled the existing drives it wouldn't be used. you'd be better off using the 1tb drive seperately and doing some spindle management (moving apps/data to different drives to avoid contention).