Adding it as a hot space should be pretty easy. Just put in the drive, open the array cards utilities (might require a reboot and going into the cards bios) and marking the new drive as a hot space.
Adding it as a part of the array may be harder (if it is possible). Not all RAID cards allow you to do this. If it does again open the utilities for the card and tell it that the new drive is a part of the array. The data will then be re-striped across all the disks, and parity will have to be recalculated. This will take a long time to <a href=”http://www.dynamic-disk.com/resource/add-drive-to-raid-5.html”>add drive to RAID 5</a>
If the card doesn’t support adding another disk into the array then you’ll need to backup all the data, break the array recreate it with the new disk, and restore all the data.