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No, it is not currently possible to read raw XML files and/or wikipedia dumps. But you could easily convert xml wikipedia dumps to AARD dictionary via aardtools: http://aarddict.org/aardtools/doc/aardtools.html
I dont see the reason to support raw xml files. You have to tell GoldenDict how to understand those files. On the other hand GD supports XDXF an xml-based dictionary format.