As far as I know you cannot (easily) use your own OS in QNAP. You can use your external 8TB drive also for the NAS. Attach it via USB to the NAS and expose OT via SMB as your other shares. But in your case I would not do this. Geht the NAS and use it and use your 8TB drive as it is right now. Thats imho the easiest and good enough way.