OK, I got it !
Xref (Pivot Table) and Pivots Filters are just awesome ! exceptionnals features ...
I have played with a lot today  

 Easy to implement mutiple Xref to the main Table.
- 1 - add the new Relation (FK) you want to associate in the xml definition of the main table.
admin/models/forms/conference.xml
<field name="themes"
	alias="id"
	label="JDPROGRAM_FIELD_LES_THEMES"
	filter="array"
	type="relation"
	multiple="true"
	labelKey="nom"/>
- 2 - add this new Relation into the view
admin/views/conferences/view.html.php
// Load relations in the object
     $model->loadRelations('intervenants');
     $model->loadRelations('themes');
- 3 - add the new PivotModel
admin/models/conferences.php
 		$this->belongsToMany('themes', // name
			'themes', // foreignModelClass
			'id', // localKey
			'id', // foreignKey,
			'coverfields', // pivotModelClass,
			'conference_id', // pivotLocalKey
			'theme_id', // pivotForeignKey
			array('logo', 'nom') // selectFields
		);
- 4 - add the new cells in the collection
admin/views/conferences/tmpl/default_grid.php
// Column title
<th style="text-align:left">
	<?php echo JText::_("JDPROGRAM_FIELD_THEMES"); ?>
</th>
...
// datas displaying images ...
	<td style="text-align:left">
		<?php
		if (isset($row->themes))
		foreach($row->themes as $rel):
                        echo JDom::_('html.fly.file', array(
                            'width' => '38',
                            'height' => '38',
                            'dataObject' => $rel,
                            'dataKey' => 'logo',
                            'indirect' => false,
                            'root' => '[DIR_THEMES_LOGO]'
                            ));
                    endforeach; ?>
		</td>
or
// datas displaying badge name ...
<td style="text-align:left">
	<?php
	if (isset($row->themes))
	foreach($row->themes as $rel):?>
		<div class="badge">
			<?php echo($rel->nom); ?>
		</div>
	<?php endforeach; ?>
</td>