If you have your own JTA Locator you can easily use it with JPOX. JPOX defines a TransactionManagerLocator interface and you need to implement this.

package org.jpox.jta;

import javax.transaction.TransactionManager;
import org.jpox.ClassLoaderResolver;

public interface TransactionManagerLocator
{
    /**
     * Method to return the TransactionManager.
     * @param clr ClassLoader resolver
     * @return The TransactionManager
     */
    TransactionManager getTransactionManager(ClassLoaderResolver clr);
}

So you need to create a class, MyTransactionManagerLocator for example, that implements this interface.

Once you have this implementation you then need to make the class available as a JPOX plugin. You do this by putting a file plugin.xml in your JAR at the root of the CLASSPATH. The file plugin.xml will look like this

<?xml version="1.0"?>
<plugin id="mydomain.mylocator" name="JPOX plug-ins" provider-name="My Company">
    <extension point="org.jpox.jta_locator">
	    <cache name="MyLocator" class-name="mydomain.MyTransactionManagerLocator"/>
    </extension>
</plugin>

The only thing remaining is to use your JTA Locator plugin. To do this you specify the PersistenceManagerFactory property org.jpox.jtaLocator as MyLocator (the "name" in plugin.xml).