Bad idea. First of all, if you can sneak in the additive, you can probably sneak in a substitute urine, whether synthetic or from a clean donor. Read the substitution sticky. Pre employment tests are not observed unless you give them cause (wow if I had a dollar for every time I say this). Next, many additives don't work on instant tests because they need time to work. When a sample is to be analyzed using immunoassay machines followed by GC/MS for confirmation, they usually collect the sample at one place then send it to the lab. Some of these additives need the shipping time to work correctly. Instant tests don't give you this time obviously. Also, some can be detected by validity testing, depending on what the additive is or how extensive the test is.