Many modelling techniques tend to address "late-phase” requirements while many critical modelling decisions (such as determining the main goals of the system, how the stakeholders depend on each other, and what alternatives exist) are taken during early-phase requirements engineering. The i* modelling framework is a semiformal agent-oriented conceptual modelling language that is well-suited for answering these questions. This paper addresses key challenge faced in the practical deployment of agent-oriented conceptual modelling frameworks such as i*. Our approach to addressing this problem is based on the observation that the value of conceptual modelling in the i* framework lies in its use as a notation complementary to existing requirements modelling and specification languages, i.e., the expressive power of i* complements rather than supplants that of existing notations. The use of i* in this fashion requires that we define methodologies that support the co-evolution of i* models with more traditional specifications. This research examines how this might be done with formal specification notations (specifically Z).