Have you looked inside your case? You speak as if you believe there is ONE drive unit inside set up as a RAID0 drive. But RAID0 is an ARRAY of TWO drives that appear to be only one as far as Windows is concerned.
If you have a fully functioning RAI0 system you probably can install in your machine a third new drive unit (as long as it has a controller port available on the mobo) but it will NOT be part of a RAID0 array, nor should it be. If you just want to use it as additional storage and keep the old RAID0 array running as it is, there's not much complexity there. Read the disk maker's instructions on exactly how to install and prepare a new drive for data storage, and use any software utilities they make available to help you. On the other hand, if your plan is to migrate all your stuff to the new drive so that it takes over as your C: drive, and then stop using the old pair in the RAID0 array (OR alternatively, begin using those drives as the extra storage) there MAY be a small issue to watch for. Moving from one drive to a much larger new one is a common task and manufacturers of new drive units love to help you with it. They usually will give you (a free download from their website) the software tool to CLONE your old drive to your new one. This is basically making a COMPLETE copy of everything and adjusting the new drive to take over as the boot disk with new extra free space on it. The wrinkle you should check on is simply that making a clone FROM a RAID0 array TO a standalone single disk requires a small change in a few parts of the disk structure data. I expect the cloning tools available are fully capable of this, but just check that detail to be sure.