Can a Muslim girl convert to Hinduism?

The is no such thing as conversion to Hinduism. One can "be" a Hindu by simply following Hindu traditions and rituals. Read scriptures such as the Vedas, the Bhagavad Gita and etc.

If we accept the United Nations Universal Declaration of Human Rights, as I believe we should, then yes. Christians, Muslims and people of all religions are free to convert to Hinduism if they wish, and the reverse. Article 18 says:
"Everyone has the right to freedom of thought, conscience and religion; this right includes freedom to change his religion or belief, and freedom, either alone or in community with others and in public or private, to manifest his religion or belief in teaching, practice, worship and observance."