Companies are increasingly engaging in complex inter-organisational networks of business and trading part- ners, service and managed security providers to run their operations. Therefore, it is now common to outsource critical business processes and to completely move IT resources to the custody of third parties. Such extended enterprises create individuals who are neither completely insiders nor outsiders of a company, requiring new solutions to mitigate the security threat they cause. This paper improves the method introduced in Franqueira et al. (2012) for the analysis of such threat to support negotiation of security agreements in B2B contracts. The method, illustrated via a manufacturer-retailer example, has three main ingredients: modelling to scope the analysis and to identify external insider roles, access matrix to obtain need-to-know requirements, and reverse-engineering of security best practices to analyse both pose-threat and enforce-security perspectives of external insider roles. The paper also proposes future research directions to overcome challenges identified.