Request Additional Fields, can they be linked to Departments

In ServiceDesk v7 7001

We are trying to have Requesters input as much information about their issue as possible. As our company branches over varying sites and departments we were hoping to have a Pull Down field where our users/requesters can identify what department they are in and which site they are located at.

I realise it is possible to create new fields in the Request - Additional Fields sub menu but is it at all possible to link the entries i have listed under "Sites" and "Departments" under the Helpdesk menu available for selection when I create a new field?

If not, what is the point of having these two seperate sub sections where we can define sites and properties if we can not allow them to be selected for identification when a request is made? Instead I am forced to go through and input a whole other listing of our sites and departments in a field for their chosing.