How to wrap an Ant build with Maven?

You can use the maven-antrun-plugin to invoke the ant build. Then use the build-helper-maven-plugin to attach the jar produced by ant to the project. The attached artifact will be installed/deployed alongside the pom.
If you specify your project with packaging pom, Maven will not conflict with the ant build.

In the example below, the ant build.xml is assumed to be in src/main/ant, have a compile goal, and output to ant-output.jar.

<plugin>
  <artifactId>maven-antrun-plugin</artifactId>
  <executions>
    <execution>
      <phase>process-resources</phase>
      <configuration>
        <tasks>
          <ant antfile="src/main/ant/build.xml" target="compile"/>
        </tasks>
      </configuration>
      <goals>
        <goal>run</goal>
      </goals>
    </execution>
  </executions>
</plugin>
<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>build-helper-maven-plugin</artifactId>
  <version>1.3</version>
  <executions>
    <execution>
      <id>add-jar</id>
      <phase>package</phase>
      <goals>
        <goal>attach-artifact</goal>
      </goals>
      <configuration>
        <artifacts>
          <artifact>
            <file>${project.build.directory}/ant-output.jar</file>
            <type>jar</type>
          </artifact>
        </artifacts>
      </configuration>
    </execution>
  </executions>
</plugin>