No, niacin will not help you pass an opiate test. The "flushing" that results from niacin is NOT "flushing drugs out of your system." It relates to an uncomfortable sensation of heat and other skin sensations.
No, niacin will not help clean drugs (any drug) out of your system. It is totally useless for that purpose. Moreover, massive doses of niacin can put you in the hospital, and can cause liver damage and even death.
last I checked niacin will only help clear your system faster, although if it was like early 1990's niacin would fool the test. now a days it does nothing and some tests will either test for niacin and goldenseal, or give a false positive with eitherr in your system.