Constructing and maintaining semantic mappings are necessary but troublesome in data sharing systems. While most current work focuses on seeking automated techniques to solve this problem, this paper proposes a combination model for constructing extensible mappings between XML schemas. In our model, complex global mappings are constructed by first defining simple atomic mappings for each target schema element, and then combining them using a few basic operators. At the same time, we provide automated support for constructing such combined mappings. Categories and Subject Descriptors H.2.3 [Database Management]: Languages ? Query languages; H.2.5 [Database Management]: Heterogeneous Databases. General Terms Management, Languages Keywords XQuery, Mapping, Extensibility, Automated support.