Thank you Michael, I'll test it.

In the meanwhile I've found this workaround:

<extension name="Add new OB call to conference">
      <condition field="destination_number" expression="^ADD_THIRD$">
        <action application="set" data="res=${bgapi uuid_broadcast ${uuid_B} local_stream://moh}" />
        <!-- ask caller for a number + #, collect into ${target_num} variable --> 
        <action application="play_and_get_digits" data="4 11 1 5000 # ivr/ivr-enter_destination_telephone_number.wav ivr/ivr-that_was_an_invalid_entry.wav target_num \d+"/>
        <action application="set" data="res=${bgapi uuid_break ${uuid_B} }" />
        <!-- add this call to the conference -->
        <action application="execute_extension" data="ADD_CALL_TO_CONF_${target_num}"/>
      </condition>
    </extension>


uuid_broadcast and uuid_break do the trick!