module.config.php

[
  'doctrine' => [
    'entity_resolver' => [
      'orm_default' => [
        'resolvers' => [
          'Module\\Entity\\EntityBInterface' => 'OptionalModule\\Entity\\EntityB',
        ]
    ]
  ]
]

Module\Entity\EntityA

<?php
namespace Module\Entity;

/**
 * @ORM\Entity
 */
class EntityA
{
    /**
     * @ORM\ManyToOne(targetEntity="Module\Entity\EntityBInterface", fetch="LAZY")
     */
    protected $marketingPackage = null;
}

Module\Entity\EntityBInterface

<?php
namespace Module\Entity;

interface EntityBInterface
{
}

OptionalModule\Entity\EntityB

<?php
namespace OptionalModule\Entity;

/**
 * @ORM\Entity
 */
class EntityB
{
   //Fields 
}

testscript

<?php
$entityA = $entityARepository->find(1);
?>

Now I get the following error message:

The class 'OptionalModule\Entity\EntityB' was not found in the chain configured namespaces Module\Entity