Additional drive use within a 4 bay LS unit

[bw37]

Model LS441, firmware 1.81-0.03 with 2 x HGST 3TB drives in Raid1

Question:  Can a third drive be installed within my LS441 to be used exclusively for back-ups of the existing raid array?
Neither the documentation (manual or FAQ's) nor linked videos address this. I think I'm learning from other posts here, that it would be possible. I understand that this would not be as safe as a separate USB drive as there are a few more ways I could lose all of it, but it would not be that different than having a usb drive sitting right next to the 441 as I have now.

My plan would be to use the extra drive within the 441 to do regular back-ups (possibly daily) and then to back up to an external drive at longer intervals.

Is this possible, and if so how do I do it (briefly...)

Thanks for your help.