The 5G-Transformer (5GT) project proposes a network function virtualization (NFV)-/software-defined networking (SDN)-based architecture to manage the end-to-end (E2E) deployment of composite NFV network services (NSs), which may involve multiple administrative domains (ADs), and hence require NS federation (NSF) capabilities. At the architectural level, this article presents the service federation functionality of the 5GT service orchestrator (5GT-SO). It covers the gaps identified in European Telecommunications Standards Institute (ETSI) NFV reports and specifications [e.g., Interfaces and Architecture (IFA) 028]. Some recommendations are also presented based on this experience, particularly on the relevance of multidomain resource orchestration. Experimental results show that the federated service under evaluation is deployed in under 5 min. The time profiling of the various processing federation-related operations shows its reduced impact in the experienced deployment time. A comparison of service deployments of increasing complexity also offers valuable insights.