adding new disk on RAID5 machine

I have a IBM x-series server here in the office. It uses RAID 5 with 3 SCSI hard disks with similar make and model. I need to expand the HDD by adding in maybe 1 or 2 more SCSI hard disks. My question is, can I just add in the new hard disk just like that and expect the controller card to do the rest of the job without over-writing the data already in the existing hard disks ? If it is not, how can I configure the RAID to include the new hard disk without damaging my data?