If you enable the network sync option that will keep the folder up to date, but since we are talking about a sync option, it will also delete the files that you delete from the NAS.
The network sync works fairly well with a few limitations. You can only sync one location at a time and the sync only runs when the Hub is in standby. As the network transfer is pretty slow you need to leave the Hub in standby for a long time the first time you sync. It will copy any new content you put into the original location, but it may take a while to show on the Hub as you have to wait until the Hub is in standby for the sync to start. I’m not sure if the sync will delete files from the Hub if they’re deleted from the original location. I’m also not sure if the sync will update any files that already exist.
Out of interest do you have backups of your stuff on the NAS. If not I wouldn’t delete your original files from the NAS. Keeping your only copy on the Hub isn’t a good idea. All disks will fail and in the case of the Hub it’s worse because the disk isn’t accessible without cracking open the case.