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Expansion of the DC distribution networks in the future power system has raised concerns over their stability assessment. To assess stability of the expanded DC distribution networks, the conventional zone aggregation method requires access to not only the nodes connected to the converters but also some other nodes within the DC passive network. However, access to the nodes within the DC passive network is not always feasible as some parts of the DC network may be buried or encapsulated. To address this issue, this paper proposes a new zone aggregation method for impedance-based stability assessment of the expanded DC-distribution networks. Compared with the conventional method, the main advantages of the proposed method are as follow: i) it only utilizes the impedance/admittance information of the nodes connected to the converters. Moreover, all the impedances/admittances can be measured directly without knowing the internal structure and/or parameters; ii) it can assess stability of expanded distribution networks with minimum number of nodes. Performance and effectiveness of the proposed method are evaluated and validated by simulation studies on a 10-bus DC distribution network and a modified IEEE 37-bus system in the Matlab/Simulink software environment. [ABSTRACT FROM AUTHOR]