Without knowing the details of the NAS, its impossible to talk in full details.
I'd say that your best bet for making this work is using it as an iSCSI disk connected to the server, which can then allow the server to use the storage as if it was local to the server.
HOWEVER there are a few issues with this:
- You are either shipping iSCSI traffic across your main LAN, which has the potential for both security and operational problems...
- ...OR you're multihoming the networking on a DC to connect to the NAS on a dedicated network link, which will cause all kinds of problems; DCs don't like to be multihomed.
- You're talking about users losing connection to the files on the NAS needlessly if there's a temporary issue with the DC.
Instead, if this is a decent NAS, it should be able to "join" the domain as a file server in some capacity and share files directly, using security groups and settings from the domain without too much trouble.
If your NAS supports joining the domain this is what you should do. If it's not the kind of NAS that can handle joining a windows domain directly then it's probably not a very good NAS (being able to integrate into an existing network is fairly basic functionality on any competent business NAS these days) and even if such a low-end device claimed to support iSCSI I'm not sure I'd trust it.