Nowadays we participate more in meetings. During meetings, information is often distributed among participants and Information Systems (IS). At the end of a meeting, only reports or formal documents are stored in the IS. However, informal knowledge exchanged is important and we wonder how to take into account informal knowledge in IS? In our work, we are developing means that allow reserving informal exchanges produced during a meeting like any document resources. Therefore, it could be possible to better understand the produced report by consulting the different comments exchanged. Our application domain is brainstorming meeting around a graph. We consider informal exchanges during meetings as annotations concerning the discussion's target (node, cluster). We developed a model and an IS prototype to that end. In this context, this paper deals with how to take into account informal knowledge in Information Systems.