Add to RAID disk pool??

I have attempted to add a 4th hard drive to an array. My server is a Dell Poweredge 2800 with 3 140GB Maxtor 10K Atlas SCSI hard drives in a RAID configuration. I have a PERC SCSI adapter.

I just need some suggestions for adding this drive to the RAID disk pool. I have searched evrywhere for a guide or something but to no avail.

Thanks in advance,
Chris