It's an ethical grey area to be sure, it all depends on much of a purist you are. I seem to recall some famous documentary getting into trouble when they lifted crowd scenes from a different 60's Civil Rights event than the one being discussed at that moment even though there was nothing particularly misleading or inaccurate in the depiction.
It happens all the time in a WW2 documentary, for example -- the doc talks about the Nazis invading Poland and they cut to b&w footage of German tanks rumbling down a road that may or may not have been from the invasion of Poland.
Today you see many mass-produced docs for TV that mix reenactments with stock footage to tell the story.
I'd talk to a documentarian...
I went to a roundtable discussion on the use of reenactments over at the Academy with Werner Herzog in the group and at some point he said "I'm not interested in an accountant's version of the truth" meaning that it was more important that the documentary be dedicated to the inner truth of the situation rather than be strictly factual in terms of the footage. But another documentarian disagreed with him, felt that you shouldn't lift footage from a different event as if it were shot for the event being discussed.