One may observe voluntary fasting before making up for missed fast days if you are pretty sure that you will be able to make up for the missed fast days before the coming Ramadan.
Dr. `Ali Sayyed Ahmad, professor of Qur’anic exegesis and its sciences, Al-Azhar University, states that: It is permissible to observe voluntary fasting before making up for the missed days of Ramadan, if you are pretty sure that after the voluntary fasting you will still be able to observe the obligatory fasting (making up for the missed fast days).
However, if the time of observing voluntary fasting coincides with that of the obligatory one, say, for instance, you will not be able to observe voluntary fasting before another Ramadan is due, you have to observe the obligatory fasting first.
What I have just said about the permissibility of observing voluntary fasting before making up for the missed days in Ramadan is based on the opinion of scholars who maintain that it is not necessary to make up for the missed days in Ramadan in haste, that this could be done at any other days after Ramadan.
Thus, you can observe voluntary fasting as long as it does not coincide with the obligatory one.