Is it permissible to add extensions to a fiancée's black hair that does not grow long, so that it becomes long as the fiancé is accustomed to, and her femininity and beauty increase, without displeasing Allah?

It is not permissible for a Muslim woman to attach other hair to her hair, and the Messenger of Allah, peace and blessings be upon him, forbade that, and cursed the one who attaches hair (al-wasilah) and the one who has it attached (al-mustawsilah). Hair extensions are forbidden whether with the husband's consent or at his request. The appropriate course of action is to treat the original hair with a specialized doctor.