SHAFAQNA – The Grand Ayatollah Makarem answered a question about vowing to fast.
Question: Can a person who must do Qadha fasting, vow to fast (Nadhri/Nazri fasting in fulfilment of a vow)?
The Grand Ayatollah Makarem: The person can vow (Nadhr) Mostahab fast, on the condition that after Qadha fast, can fulfil it. Meaning that, if for example has ten days of Qadha fast and there are more than ten days left to the next Ramadhan, can make a vow, but if only ten days left, considering that must do Qadha fast; there is a problem with vowing Mostahabi fast.
Source: PERSIAN SHAFAQNA