Forget the additives.

Drop-in additives will not work if its an instant-type test because it takes up to 90 min to fully destroy the drug metabolites in the urine sample.

Also, the labs are detecting additives on a more frequent basis than before. They are detected when the urine sample gets flagged for excessive nitrites, high chromates, and/or an abnormal pH.

Since youre entertaining the thought of sneaking something into the test facility, you might as well sneak something in thats going to work - a clean persons sample or synthetic to use as a substitute.
Its much more easier to pour a sub sample into the cup, rather than holding the cup in one hand and fumbling around with additive vials in the other.