The premixed stuff is not going to work because it is completely chemical free, and you can't add any of the drugs to it either. This is a long shot,
If you have a friend willing to help you out, depending on what drugs you are taking that need to show up in the test. On the day of the test have your friend take the meds, and use his urine . You can dump out the fake stuff wash the bottle out with super hot soapy water, then about an hour before the test have your friend pee in the bottle. But if you have been taking prescribed medicine on a daily basis you will have a lot of it in your system therefore I am not sure if one day's dosage would be enough to show up on your friends urine. I don't know what meds your on but if there are side effects you can't ask your friend to go through all of that for you.
I know what it feels like though desperate times require desperate measures.