Is it permissible to give the building's doorkeeper, who was promised charity, a portion of the Zakat after the year has passed, to help him bring his wife and rent accommodation for her?

If the guard is eligible to receive zakat, there is no harm in giving him zakat, even if you had previously intended to give him a general charity, as long as you have not yet given it. What is prohibited is giving money with the intention of charity and then changing the intention to make it zakat after the payment has been made.