Expanding NVR Storage From 2 To 4 Weeks?

I have customer who has system with 8TB of storage on 12 cameras.There a mix of 3 and 4mp set on 1080p at 25 fps. System gets about 2 weeks at present. He wants additional 2 weeks of recording at roughly same quality. I can do some tweaks but gaining 2 weeks is kinda stretching it. What options are out there to expand without going from 16 channel to 32 channel nvr? I know there is nas storage but how does it integrate with present system?

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