Thanks to Lodder's answer and some help from the [french Joomla forum][1], this is what i’ve done :

 - Make a backup of the Joomla 2.5
 - Install the backup
 - Upgrade to Joomla 3.x

**Copy the requrired tables :**

_comprofiler_xxx (All the table community builder)

_users

_usergroups

_user_usergroup_map

That's it !

  [1]: http://forum.joomla.fr/