Yes. Just add the drive in. Unless you go into the RAID and add it to the RAID (Which you wouldn't be able to. You would have to break the RAID 0 first then remake it with 5 drives in which you will have to backup your whole PC and then restore it) it will just see it as an extra drive.
Then use something like Macrirum Reflect and clone just your OS partition to the new 120GB SSD or just reinstall windows to it (If you do that you will want to unplug the power to all the RAID 0 drives BEFORE you install windows then plug them back in after and set primary boot to the single SSD).
If you clone you MAY have to make a Macrium Reflect Rescue Disk, boot off of that and fix the MBR (boot off of disk, then click on Restore and then Fix Windows boot problems, select the 120GB SSD and next and you should be good).