Thanks, everyone for the responses! I did forget to mention that I bought one extra new drive, the same as my previous 2 HDDs (4TB each), and that's the one I'm wanting to add (so I would have 3 x 4TB in my pool, now with room to grow). It looks like backing up my current data, wiping the pool, then restarting with this new setup would be the easiest method here.
Thanks again!