I wonder what would have happened if let the sink drip enough water to make up the difference?
Before you think about doing this, youd better verify the urine samples specific gravity and creatinine levels and hope that they are well within the acceptable ranges.

What can happen is the water can cause the creatinine and SG numbers to take a nosedive, resulting in a "too-dilute" ruling. And if the numbers happen to drop low enough, the result would be a "not consistent with human urine" (aka "substituted specimen") ruling.

I wouldnt recommend adding anything or "topping off" a drug-free substituted sample. 2 fl oz of sample is enough for a U/A. The collector may ask for more, only to discard the excess. Under DHHS regulations, if 2 fl oz of urine sample is all the donor can submit, the collector has to accept it.