I don't think there's an OMV NTP server plugin, however, what you want can be done on the command line. While there's a bit of reading involved, the actual configuration and setup isn't very hard. NTP Server
I'm trying to understand your use case for doing this. To use NTP, it's necessary to be connected to the Net. Since nearly all network aware devices have an NTP client built in, their time would be more accurate if they connect directly to a time server pool. (The NTP protocol is designed for this.) Using a proxy server introduces an additional, potentially unnecessary, element.
Thanks to all yours responses. only yesterday bought a rtc for the pi, configured it and its working flawlessly. Installed requeried packages according to the submitted link (Thanks flmaxey!) and its working fine so far.
i want this kind of setup because the raspi is connected to a LAN with no internet access.