Thanks to Lodder's answer and some help from the french Joomla forum, 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 (car celui qui avait créé le site avait créé des nouveaux groupes d'utilisateurs)

_user_usergroup_map

That's it !

Thanks to Lodder's answer and some help from the french Joomla forum, 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 !