Is what I did, helping a female colleague at work by registering the project in my name to transfer money to her, knowing that I would not do the work, considered unlawful (haram) due to lack of integrity with the employer? And how can I expiate this sin?

It is not permissible for you or your friend to act against the wishes of your employer. Your aforementioned action is an aggression and a deception, as the Prophet, peace and blessings be upon him, said: "Whoever deceives us is not of us, and scheming and deceit lead to Hellfire," and his saying about the people of Hellfire: "And a man who neither wakes up nor goes to sleep but that he is deceiving you regarding your family and your wealth."

You must repent to Allah and return what exceeded the wage to the employer, unless he forgives you. For repentance concerning the rights of people necessitates returning injustices or seeking forgiveness from the rightful owner.