In networked embedded systems, runtime adaptive software promises an increase of flexibility, fault tolerance and extensibility. Often, this requires that software components have to be allocated dynamically to execution platforms at runtime. Hence, the platforms have to execute dynamically changing task sets. However, in real-time systems, a task set cannot be executed without previously checking its schedulability wrt given timing constraints. Therefore, it has to be determined, whether or not the existing task set would be still schedulable including newly arriving tasks. In this paper, we propose a schedulability test algorithm for such systems, exploiting the situation of adding a new task to an existing task set. Therefore, we adapt existing schedulability tests to exploit the specific acceptance test use case. The benefits of the developed adaptations are shown by experimental investigations.