The current static job scheduling on supercomputers for MPI-based applications is well known to be a limiting factor for the exploitation of a system's top performance in terms of application throughput. Hence, allowing fully flexible and dynamically varying job sizes would provide multiple advantages compared to the current approach, e.g., by prioritizing jobs dynamically and optimizing resource usage by transferring resources economically. A critical step in achieving dynamic resource management with MPI on supercomputers is the development of sound and robust interfaces between MPI applications and the runtime system. Our approach extends the concept of MPI Sessions, a new concept introduced with MPI 4.0, by adding new features to support varying computing resources via the MPI process set abstraction. We then show how these features can be used, as a proof of concept, to request (active) and cope with (passive) varying resources from an application's perspective. To validate of our approach, we develop libmpidynres, a C library providing an emulated MPI Sessions environment on top of existing MPI implementations without MPI Sessions support, which we then use to integrate our proposed extensions to the interface specification. Using this proof-of-concept environment, we show how an MPI Sessions enabled application can use process sets to handle dynamically varying resources.