Add disks to a Mega raid

We have a windows 2000 server that has a Mega raid system with three disks set to raid five. I have ordered two disks that are the same size and speed but not the same manufacture. In the Raid set up it will show the original raid setup and it shows the two new disks. To add the new disks to the original raid do I edit the original to add the two new disks. How do I do this? I am a bit nervous as I cannot loose the data on the original disks. I will be doing a full back up.