The WTO procurement procedure for IT infrastructures and services involves the creation of software requirements specifications (RS). This task is challenging for the public administration, as few aids and guidelines are available. But RS is important in the WTO procedure because it serves as a basis for the evaluation of bidders. On the other hand, BPMN is increasingly applied in administrations to document business processes. Taking the situation of the Swiss administration as a reference, we introduce an original methodology to specify requirements from business processes in BPMN 2.0, based on standardized phrase patterns that are attached to BPMN elements. Our methodology also provides a template structure and guidelines for producing RS that can be included in WTO- conform Request for Proposals. Our methodology originates from the situation in Swiss administration, but it is generic enough to be applied in other public administrations as well as in private companies. We also conduct user interviews to identify the actual tasks and challenges faced by users when specifying requirements. Findings from the interviews suggest that communication issues between stakeholders (business users, analysts, IT specialists) are the most pregnant. Based on results and analysis of user interviews, we design a software prototype that aims at solving most problems faced by users when producing requirements specifications. The main advantage of our methodology is that administrations can reuse existing BPMN process documentation as a basis for specifying requirements in the WTO procedure.