Please take a look at the HP Smart Array Reference Manual and its section on Array Expansion and Migration.
In short, you have the concept of an Array (the assembly of the physical disks) and a Logical Drive (logical RAID arrays within the physical Array).
Your current setup has a Logical Drive of 1TB in RAID 1. Your unused space can be used to extend the existing Logical Drive, or to carve out a new Logical Drive.
If you are doing this using the HP Array Configuration Utility (web interface), you can follow the prompts to expand the array. The space will be available to Windows, but you'll have to handle the partitioning at that level.
This also depends on your controller's setup, though. The Smart Array P212 controller can only do this online if you have a battery-backed cache (BBWC) unit installed.