Never get an answer in this forum !!!
For others who have the same problem : the solution is to replace the JBoss embedded Tomcat, by a new version of Tomcat. This can be done by downloading JBoss 3.2.3 on sourceforge !
In : "jboss-3.2.3\docs\examples\tomcat\" there is an Ant build which allows you to create a new configuration of JBoss (all|default|min) with Tomcat 5.0 embedded. Then Tomcat 5.0 can then be launched as a JBoss service (in the same JVM -> local communications between servlets and EJBs !!!)
Bye all !