Here is an example of how a POM for a Maven 1.x plugin would look. This is for the Maven 1.x site plugin:
<project>
<modelVersion>4.0.0</modelVersion>
<groupId>maven</groupId>
<artifactId>maven-site-plugin</artifactId>
<packaging>maven-one-plugin</packaging>
<version>1.6-SNAPSHOT</version>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-one-plugin</artifactId>
<version>1.3</version>
<extensions>true</extensions>
</plugin>
</plugins>
</build>
</project>