Modern User Interfaces need to dynamically adapt to their context of use, i.e. mainly to the changes that occur in the environment or in the platform. Model-Driven Engineering offers powerful solutions to handle the design and the implementation of such UIs. However this approach requires the creation of an important amount of models and transformations, each of them in turn requiring specific knowledge and competencies. This leads to the need of an adapted tool sustaining the designers' work. This paper introduces UsiComp, an integrated and open framework that allows designers to create models and modify them at design time as well as at runtime. UsiComp relies on a service-based architecture. It offers two modules, for design and execution. The implementation has been made using OSGi services offering dynamic possibilities for using and extending the tool. This paper describes the architecture and shows the extension capacities of the framework through two running examples.