The NI-447x devices do not accept external clock due to the delta-sigma converters they use to realize high dynamic range characteristics. Check out this KB which has some more details:
http://digital.ni.com/public.nsf/websearch/862567530005f09c8625668d00751347
So, no, the trigger input can not be used as a external clock input.
Are you trying to synchronize the NI-4474 with another device? What type of synchronization do you need? What type of device are you trying to synchronize with?
-Jack