First of none of the volunteers who came to help Israel in 1967 were involved in fighting. They replaced Israelis who were in the military on kibbutzim, in hospitals and community service so that the country could carry on running as normally as possible. But in principle as long as those who came to help Israel did not fight against their own countries I see no problem with this. In 1948 many who came were involved in the fighting. Did not Arab fighters pour in from Iraq and other Arab countries to help kill the Jews in 1948? And no, they are not mercenaries unless they are impartial hired professional soldiers paid to do the job.
U.S. defense contractor accused of copying secret military documents (Reuters)
from the article: Magnate in U.K. scandal: I feared charges of a 'Jewish conspiracy'