I think the antrun plugin is the only way to include sources from outside of the mvn project for generating the XML schema.
<plugin>
<artifactId>maven-antrun-plugin</artifactId>
<version>1.6</version>
<executions>
<execution>
<phase>generate-resources</phase>
<configuration>
<target>
<taskdef name="schemagen" classname="com.sun.tools.jxc.SchemaGenTask">
</taskdef>
<schemagen destdir="src/main/resources/" includes="<comma separated paths to include>"
excludes="<comma separated paths to exclude>">
<src path="src/main/java" />
<src path="../OtherProj1/src/main/java" />
<src path="../OtherProj2/src/main/java" />
</schemagen>
<move file="src/main/resources/schema1.xsd" tofile="src/main/resources/<filename>.xsd" />
</target>
</configuration>
<goals>
<goal>run</goal>
</goals>
</execution>
</executions>
</plugin>