Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
New Muslims are not required to make up missed acts of worship before accepting Islam.
Answering your question, The Permanent Committee for Scholarly Research and Ifta’ in Saudi Arabia, states:
One who accepts Islam is not to make what he/she has missed from prayers, fasting, zakah before becoming a Muslim. Allah the Almighty says:
{Say to those who have disbelieved that if they cease, what has previously occurred will be forgiven for them…} (Al-Anfaal 838)
Prophet Muhammad (peace be upon him) said: “Islam annuls what came before it.” (Muslim)
The Prophet did not command anyone among those who accepted Islam to make up for anything that he previously missed from the rites and rituals of Islam during his days of disbelief. The scholars of Islam are in consensus about this.
Almighty Allah knows best.
Source: www.islamqa.info.