As the number of high voltage direct current (HVDC) installations is increasing, there is a growing need for interconnection to create more efficient grids. To tackle the main challenges in HVDC grids, a multiport DC-DC converter, often referred to as DC hub, is considered a solution. This paper provides a thorough review of different DC hub design approaches and examines the modularity of existing DC-DC converter designs to allow the connection of more ports. A design based on multilevel modular converters (MMC) is chosen due to their inherent advantages and its control operation is described. A DC hub was modelled in Matlab/Simulink and was tested in a case study including the interconnection of three DC lines in symmetric monopolar configuration, operating at different voltage levels. The simulations showed that the DC hub can maintain controllability under different operating conditions and can assist the power flow control of an HVDC grid.