In the following example, each source file with file name ending with -abc.xml will produce a destination file with file name ending with -abc-out.xml.
The -out part will effectively be inserted into the file name during the transformation.
<execution>
<id>transform-abc</id>
<goals>
<goal>many-to-many</goal>
</goals>
<configuration>
<xslTemplate>path/to/template.xsl</xslTemplate>
<srcDir>src/main/xml</srcDir>
<srcIncludes>**/*-abc.xml</srcIncludes>
<fileNameRegex>-abc\.xml</fileNameRegex>
<fileNameReplacement>-abc-out\.xml</fileNameReplacement>
</configuration>
</execution>