if you just want to add it as a non-RAID drive, yup, just plug it in (as long as there's a spare port)
if you want to add it as a RAID drive (spare for example), you'd have to do something in the console to make it a spare - just adding it in does nothing. after you add it in, you'll see it appear under the non-RAID drives in the console's device tree
the one caveat is that if your volume is degraded when you add the drive, it will ask you if you want to rebuild to that drive. if you don't - you just want it to be a non-RAID drive - say no.