Cells in conventional Modular Multilevel Converters (MMC) designs use a capacitor for energy storage. This means that the net power balance for each cell (neglecting losses) needs to be equal to zero, the MCC realizing therefore a power transfer between its DC and AC sides. This paper analyzes the design, operation and control of MMCs in which the cells have the capability to transfer (inject or drain) power. The use of such cells opens several new functionalities and uses for the MMC. On one hand, it would allow integrating elements like distributed energy storage (e.g. batteries), low-voltage/low power sources (e.g. PV) and loads at the cell level. Cells with power transfer capability can also be used connect the medium/high voltage DC and AC ports intrinsic to the MMC, with low voltage DC/AC ports at the cell level. This would result in multiport power converters, potential applications of this topology including solid state transformers (SST).