You can add your 40 Gb into the RAID, but it will give you less than 18 Gb usable storage. A better bet will be to put it on a different channel and use it as a separate partition. Hopefully you have a second channel on the array controller, otherwise, you'll have to find some sort of SCSI for it. I wouldn't mix SCSI and IDE storage devices on the server.
How you go about it depends a lot on the OS. Compaq has some good documentation on how to do it. It's not a big deal. Good luck.