Masking is impossible but diluting is possible however the urine temp is (SUPPOSED TO BE) checked and logged at the time of collection. The urine should most likely be close to body temp. Any liquid used for diluting should be kept under your arm pit to reproduce a similar temp.
The person is not adding liquid to a sample in an effort to dilute. The person is consuming the liquid. Therefore, when it comes the time to urinate the fluids consumed, the urine sample would be at or near the body temperature.