Is it software or hardware based RAID?

If it's hardware, you should be able to add a new drive, or upgrade each drive to a larger size (waiting for each one to rebuild first). Doublecheck with your vendor though to ensure one of these two ways works. It's how the ones I deal with on a daily basis.

If it's software, I don't have any advice for you on this. In my general experience, it tends to be less reliable if something goes wrong.

Before adding space, ensure you have two known good backups just in case.