No you can do that. The system will only use 1TB of the new disk space for the automatic RAID recovery. The SS4200 is a pretty basic RAID machine. In fact, it uses Linux software RAID (mdadm stuff). You can access the system at a root level command prompt and run teh madam configuration utility, but if you're doing root level command line stuff you really should have a backup in case anything goes wrong. If you have a backup, there's no need to do what you're thinking about.
The system should support 1.5TB HDDs from a platform perspective, but none have been validated by Intel. You're kinda on your own with anything above 1TB HDDs.