An inherent concern for a wireless sensor network (WSN) is the unbalanced energy consumption problem, where sensors closer to the sink are more likely to exhaust their energy faster than other nodes. To mitigate this problem, this paper considers including some resource-rich mobile nodes, called mobile data-pumps, to conduct data relaying from static sensors to the sink. The network thus becomes a two-tier network, with the original static sensors at the low tier and data-pumps at both low and high tiers. We propose a novel distributed navigation protocol that does not rely on any location information of sensor nodes to relocate data-pumps to meet both goals of connectivity and load balance. The main idea is a concept called virtual Voronoi cells, which can help data-pumps to locally balance their loads using the underlaying low-tier topology and thus significantly balance energy consumption of sensors. Simulation results are presented to verify the effectiveness of our result.