You can technically freeze urine to preserve it for a drug test, but there are also two problems to be wary of:
1. Getting the temperature right on the day is super important. Most drug tests will be rejected if the temperature doesn't lie between between 91 and 97 degrees fahrenheit..
2. You need to keep it air tight. This is to prevent bacteria forming on the urine, a dead giveaway if you show up with urine so toxic it would kill you.
3. Transporting it to the drug test location is a detail a lot of people neglect. It's not as easy as you think to smuggle urine into a drug test, and any suspicions will result in a retake.
So be careful!