Complex interactions between two organizations, involving sensible information and resources, requires to honor each organization's security policy. This implies to make compatible and combine different sets of policy rules that were designed for different organizations, and, therefore, different subjects, actions, and objects, classified and organized in different manners. However, finding out what is the security policy that emerges from the combination of all the organization-level policies and the higher-level interoperability policy is not an easy task. In this paper we provide a methodology based on Finite State Transducers to analyse this situation modelling policy-rules, mapping entities, combine them, and automatically generate an interoperability set of security policies