This article tackles the issue of integrating heterogeneous archival sources in one single data repository, namely the EHRI portal, whose aim is to support Holocaust research by providing online access to information about dispersed sources relating to the Holocaust (http://portal.ehri-project.eu). In this case, the problem at hand is to combine data coming from a network of archives in order to create an interoperable data space which can be used to search for, retrieve and disseminate content in the context of archival-based research. The central aspect of the work described in this paper is the assessment of the role of the Encoded Archival Description (EAD) standard as the basis for achieving the tasks described above. We have worked out how we could develop a real strategy of defining specific customization of EAD that could be used at various stages of the process of integrating heterogeneous sources. We have developed a methodology based on a specification and customization method inspired from the extensive experience of the Text Encoding Initiative (TEI) community. In the TEI framework, one has the possibility to model specific subsets or extensions of the TEI guidelines while maintaining both the technical (XML schemas) and editorial (documentation) content within a single framework. This work has led us quite far in anticipating that the method we have developed may be of a wider interest within similar environments, but also, as we believe, for the future maintenance of the EAD standard.