ON MAtters Calendar and AddressBook, you are very right, this was not possible, i am not sure why @webtop_team dev did not see the need to implement Cardav and caldav support or even just ability to sync with an existing caldav and cardav server.
Overall, the syncing can be acheived with third party tools, and its cumbersom
As discussed in this thread a while back: The Trouble In Calendars - Chat - NethServer Community
i think @mrmarkuz pointed out about using webtop as a caldav client here: The Trouble In Calendars - #6 by mrmarkuz
Mostly the Files Part.