Not familiar with the raid controllers on that board but generally speaking, it should not cause any problems. I would add the standalone drive to the other controller but that's more a preference... array(s) on one controller, standalones on another.
are you just adding this as an additional hard drive to exist on it's own? I assume that's what you're getting at. Or are you looking to add it to an array of the two other drives?
I am pretty sure you can just pop it in. Make sure your RAID controller supports soemthing call JBOD (I may have misspelled that), which I beleive is a special statues where the controller just "hosts" the drive and does not make it a member of any arrays. Do not set it up as a spare, or you could have it formatted if any of your raid drives die (although your array is RAID 0 which cannot be rebuilt anyway...)
there are some really crappy controllers out there that will not allow you to have a seperate disk running on its own, however I do not remember which ones that were. However, I would like to make you aware that there is -supposedly- a different driver for nForce4 in RAID and non-RAID configurations. You may want to research that a bit, if you use it.