A number of IEEE 802.15.4 devices can be connected by a tree topology as proposed by ZigBee specification. Address configuration in tree-based ZigBee networks needs to assign every device a network address that uniquely identifies it from others, and such addressing should also assist routing. The addressing method recommended by the specification forces a static assignment that is coupled with node's location in the tree, resulting in an inflexibility in allocating addresses. This property may significantly decrease the ratio of addressable devices and cause routing detour. To alleviate the problem, this paper considers three alternatives that manage address space with flexibility but require additional storage in ZigBee routers. Performance evaluations indicate that proposed approaches provide different levels of tradeoff between the ratio of addressable devices and storage costs in ZigBee routers.