Applications that are extensible by end-users represent a proposal to cope with the issue that it is improbable that a software will attend every specific need of each different user within a domain. One of the research areas approaching extensible applications is End-User Programming (EUP). EUP empirical investigations evidence the existence of collaborative practices related to the process of application extension. In these practices, people communicate with each other with various purposes: to help themselves deal with their problems regarding computing, to share ready-made extensions or to build extensions together. Our work complements these investigations by offering a deep and theoretically motivated treatment of the communication phenomena related to the observed collaborative practices. Based on Semiotics theory and Semiotic Engineering, we propose a model of these phenomena, describing its structure and behavior thus supporting a better understanding of the issues related to them. According to the model, we designed a computable language for the representation of extensions in association with corresponding communicative discourse.