I need to know if it's halal/haram to convert someone's money from bank. If someone has black money, by which I mean he has not paid tax to the government, can we convert it to white money (i.e. money laundering) and take 10 or 20 percentage of the total amount as a commission?
If money laundering is a crime, then it's not permissible. Muslims are required to obey the law.
O you who have believed, obey Allah and obey the Messenger and those in authority among you. ... -- Qur'an 4:59
This is confirmed by fatawa, e.g.:
Muslims must adhere to the laws of any country they live in, whether in the west or the east, as long as the law is not in contradiction with one’s religion. -- Darul Iftaa
Other examples are IslamWeb and Abu Amina Elias, and if you can't obey the law (i.e., the law clashes with Sharia law) "Muslims are required to emigrate to a land where they can practice their religion freely" (see What should I do if Canadian law and Sharia law are incompatible?)