One of the major problems of software development is to pay attention to all specific needs of each user in a domain. The proposal to use extensible applications tries to solve this problem. Extensible systems are developed so that they can be shaped by end-users, adding, modifying or removing functionalities, and so evolve in time. A research area related to extensible applications for domain expert users who are not programming professionals is that of End User Development - EUD. However, it is not found in the literature, research that specifically support the designer in making decisions about when it is interesting to extend the system and which part of it can be extended. The purpose of this work is to inform the designer about different extension opportunities related to the result of user and task analysis. It presents a classification, based on Semiotic Engineering, which encompasses the investigated techniques and the extensible applications. Then, it identifies, in the user and task analysis questions, those related to the techniques described in this classification. Therefore, the designer can identify which approaches can be used in which situations to extend the system. To evaluate this proposal, we developed a case study to re-build an authoring tool for interactive multimedia programs called Composer. The study goal was to keep the application flexible and extensible without requiring from users too much knowledge about the application´s underlying language - the NCL (Nested Context Language).