Yes, you can add hot water to a Keurig to troubleshoot a brewer that is stuck on preheating. The hot water tricks the brewer to jumpstart and bypass preheating. Adding very hot water forces the Keurig to detect it and tricks it into brewing However, it is not advisable to add hot water as a way of increasing the temperature of your coffee. Keurig's temperature can only go as high as 192° F. Therefore, adding hot water provides no extra advantage: your Keurig will still brew at its preset temperature. Moreover, the cold water tank in Keurig has no insulation and is not primed to maintain heat. Keeping hot water in the reservoir might also damage it.