A new solution is proposed to integrate Free-Space Optics(FSO) technology into the existing RF-based Wireless Mesh Networks(WMNs) in order to improve the capacity of wireless mesh networks. Firstly, based on the expected long-term uplink and downlink traffic demands at each node, an Integer Linear Program(ILP) is constructed for the joint assignment of FSO transceiver and RF link scheduling to find the key location nodes which increase the overall network capacity to the greatest extent if they get upgraded. Secondly, in order to avoid the complexity of the original ILP, a Probabilistic Greedy Scheduling(PGS) algorithm is proposed, which can calculate a feasible solution at a very low computational cost. As a comparative benchmark, a Simulated Annealing(SA) algorithm is also proposed. The simulation results show that the network capacity can be improved by upgrading some key location nodes.