Will diluting the urine for a portable drug test by adding water to the cup help?

yes if the doctor's really stupid Two ways to dilute the concentration of anything in your urine are direct dilution; by adding another liquid to the sample, and indirect; ie drinking a LOT of water just before the test. Test facilities are aware of most schemes to alter a test result and they take measures to prevent any tampering with the sample. Usually they put a blue dye in the toilet water to stop any addition of that water, and they limit the access to tap water in a sink. Additionally they often check the temperature of the sample to assure that it is close to the normal 98.6 F of a human body, in case you tried to smuggle in your own water. If you drink a lot of liquids to attempt that type of dilution they may notice that the urine is an extremely light color and reject the sample. Taking a B-complex vitamin just before the test will cause the urine to be the proper color and make the sample appear unaltered.