First of all, it is highly recommended to perform Witr, as it was practiced by the Prophet (peace and blessings be upon him) and he encouraged Muslims to practice it. The Prophet (peace and blessings be upon him) answered the one who asked him how many prayers Almighty Allah made obligatory on him saying: “Allah has prescribed five prayer in the day and the night.” The questioner said, “Am I obliged to do more?” He said: “No, unless you perform supererogatory prayers.” The man went away saying: “By Allah, I will not increase or decrease beyond these obligatory prayers.” Then, the Prophet (peace and blessings be upon him) said, “He would succeed if he did what he said.” (Reported by Muslim)
With the above in mind, we would like to stress that the Muslim scholars state that witr should be the last prayer that one performs before going to sleep. However, there are others who state that if one performs witr and after that he sees to pray, then he can do so and he is not required to repeat the witr prayer. This is based on the opinion `Ali Ibn Abi Talib according to the hadith in which he heard the Prophet (peace and blessings be upon him as saying) “There should be no two witr prayers in the same night” (Reported by At-Timidhi)
Based on what is mentioned above, we can conclude that there is nothing wrong with performing Tahajjud or other Nafl prayer according to what is mentioned above.