Any reason to move the contents to ownCloud instead of accessing to those contents through ownCloud? Moving 50+ GB might take a long time.
I'd recommend to check what the files_external app can offer to you. The main goal is to access to those files through different connectors such as SFTP, webdav and others that can be configured in the settings page. There are more connectors available in the marketplace.
Basically, check how can you access to that NAS from the ownCloud server and configure the appropiated connector accordingly.