Audio files
I'd recommend doing something like this for Audio files.
After your last rip clear the "ripping library" or create a new "ripping" library on the local PC; get a backup from your main server library and restore it to your ripping library. This will copy, identically, all your library structure views etc.
Select all files (in newly created library) - as all will have broken links and delete them from the library only. Then you will have an identical library to your master, just empty. Make sure all your important fields are marked to write to disk (Options=>Manage Library and Fields=> check Save In File Tags (if Possible) )
AFter you transfer your files to the NAS, run an autoimport and all your tagging will be preserved (that could be written to the file) and inserted in the library database.
You could also use an external ripping program, ripping the files to the NAS. And using the PC as a client, do your tagging remotely. But I don't think you are asking about that.
For Video this is not going to work as well. If you are trying to do this with video you will have to transfer the sidecar files and I'm not sure what is actually saved to these files. You will also not have anywhere near the tagging abilities you will have with flac audio files. If you are trying to do this for video, you should not do any automatic tagging as this will replace the sidecar files.Hope this helps
edit: I see this in the Mac forum now, I'm on PC but I imagine this works in exactly the same way. And forget about the video