Is it permissible to add an extra amount to the Zakat al-Fitr with the intention of charity, such as if the…

Is it permissible to add an extra amount to the Zakat al-Fitr with the intention of charity, such as if the amount of Zakat al-Fitr is 20 riyals, to set aside 25 riyals—20 of which is for Zakat al-Fitr and the remaining for charity?