Is it permissible for an employee of a charitable organization to disburse a sum from the Sadaqat (charitable funds) to himself under a fictitious name, knowing that the organization does not give him from the Sadaqat because he is one of its employees?

It is not permissible to lie to or defraud the charitable organization. What was taken or spent must be returned to its rightful recipient, along with repentance and seeking forgiveness. As for his claim that he is one of its workers, either he works for a salary and is therefore not entitled to anything, or he volunteers his work, in which case he has no right to demand anything. Moreover, there might be no need for his work.