Need additional field in my templates as common field "Name" (where we select requester).

Scenario: In our organisation users add to AD from HR application. Service desk get requesters from AD. Sometimes we have requests from departmens head, to make user rights in some of our corporate systems, but that user does'nt introduced in our HR system. I want create field, where Department head can choose who need service which he asked from request.(some request can make only dep. heads)