Documents on traditional file servers or NAS like department shares or user home drives can be centrally synchronized with Microsoft Office 365, SharePoint, OneDrive for Business, Office 365 Groups, or Microsoft Teams using the Layer2 Cloud Connector
with very flexible options. The connector can close many gaps with SharePoint and Office 365 out-of-the-box document synchronization, e.g. regarding the number of files allowed, file naming conventions, file types allowed, document set selection, metadata assignment, central management, logging, alerting, and more. Please take a look here
for more supported systems and applications like Google Drive, Dropbox, or Box.