Do you see this attribute populated on identity cube from debug page. The attribute type for Administrator attribute is identity and hence you can only set it to an identity. You will also have to make sure you update you ObjectConfig for Identity to remove administrator attribute from the map entry defaultIdentityType – > IIQ_default – > DisallowedAttributes.
A sample source mapping application rule that sets administrator attribute is shown below which can be used to set the attribute to Identity type.