[Mixing Obligatory Zakāh with Voluntary Charity]

Answer: His way out is to distribute all of it to the poor who are eligible recipients of zakāh. By doing so, the zakāh will have reached its rightful recipients, and the voluntary charity will also have reached its rightful recipients.
 He may, even after the mixing, distinguish the zakāh from the voluntary charity if he remembers the amount of each. His mixing them is a culpable act toward those funds; by that culpability he becomes liable for them. Owing to that liability, he must pay out the amount of zakāh and the amount of the voluntary charity—from what is mixed, or from other funds—because by mixing them the zakāh amount and the voluntary amount have transferred to his personal liability. And Allah knows best. Praise be to Allah, Lord of the worlds. May Allah bless Muḥammad and his family and grant them peace.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1