Google "save whatsapp video to pc" - plenty of different methods listed there, depending on what kind of phone it is, what capabilities your PC has etc. Once saved (assuming you usually access NSC via a PC), it should be very easy to attach your file using the 'insert video' link at the top of the message box on NSC (the one that appears when you hit Reply)
I've got some good ones from QPR on my iPhone if I go on to NSC on my phone can I just upload the video or do you have to do it via Twitter (which I have ) or YouTube channel (which I don't)
Did I miss a thread with videos as I've not seen any?