We consider the application of passivity theory to the problem of voltage stabilization in DC microgrids (DCmGs) given by the interconnection of Distributed Generation Units (DGUs), dynamic RL lines, and loads. We first provide a novel result on stable interconnection of multiple passive systems and then apply it to DCmGs. More specifically, we consider the decentralized controllers proposed in [1] synthesized by solving linear matrix inequalities, which allow addition and removal of DGUs in a plug-n-play fashion. We show that they passivate DGUs and combine this property with the inherent passivity of RL lines for proving voltage stability without Quasi-Stationary-Line approximations, which are assumed in [1]. Theoretical results are backed up by simulations in PSCAD.