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High bandwidth one-to-many applications emerging in IP over WDM optical networks demand multicast support at WDM layer so that data signals would be efficiently duplicated inside network without O/E/O conversion. Multicast trees in an optical layer can be built with light-trees which use light-splitting technique. Most of recent studies have been focused to efficiently build and configure light-trees without existing unicast or multicast traffic into consideration. In this paper we consider the dynamic and optimal design problem of multicast configuration for realistic and constrained WDM networks. In such a network, both unicast and multicast are supported, and WDM switches have limited number of wavelengths and light splitting capability. On the other hand, the amount of bandwidth per wavelength is abundant. Using subwavelength sharing among traffic demands of unicast and multicast, we build a hybrid virtual topology which exploits both existing light-trees and light- paths. By optimizing WDM resources in addition to resource sharing with existing unicast and multicast demands, we truly maximize the WDM layer capability and efficiently support more multicast traffic demands. We validate the efficiency of our approach with extensive simulations.