The short answer is "no". The more detailed answer is not really, unless you want a really asymmetric RAID-5, where you would throw away more than half of your 1TB drive. As Dwarfer suggested, you could change the array to a JBOD (which isn't technically a RAID array).

Things to keep in mind:
On a cheapo software RAID controller (like NVIDIA, VIA or Silicon Image) to my knowledge you cannot preserve the data during this process; you'd essentially have to backup and reformat. And of course, JBOD stands for "Just a Bunch Of Disks", meaning the system addresses it as one big drive. However there is absolutely no redundancy with a JBOD, and no speed bonus either, so it would be slower than your existing RAID-0.