All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
During office Hours, it is not permissible for you to work for your own benefit without the permission of your employer.
The employee is like a privately hired person, and the benefit of his time is owned by his employer during working hours.
Kashshaaf Al-Qinaa’ reads: "The private worker is the one whose benefit is measured by a specific amount of time, and his time is exclusively for the employer during the period of the contract; no one else can benefit from his services (i.e., time) during this period of time." [End of quote]
As for the time outside your working hours, then there is no harm for you to work.
Allah knows best.