Raid and other hard drives not in Raid (HP E200i Ml350 G5)

Hi we have a Raid 5 (HP E200i) in our ML350 G5 Server (4x146GB)  logigal drive C: is about 440.3 GB
The free space on the disk is getting low.

So is it possible to add a new drive not in the raid, only 1 more drive for unimportant data. and make it d:\
i read that the controller can handle up to 8 drives. But if you have a Raid, musst all drives be in this raid? Or can i have my existing Raid 5 with 4 drives. and additional 1 or more drives?