Recently, many organizations such as universities and companies are deploying multiple wireless local area networks (WLANs) even in the same building. In these WLANs, wireless access-points (APs) are usually managed independently by different groups such as laboratories or departments. In this overlapped WLAN environment, a host can detect signals from multiple APs, which may degrade the communication performance due to their interferences and increase the operational cost. To solve this problem, previously, we have proposed the AP aggregation algorithm to minimize the number of active APs in the field by aggregating them using the virtual AP technology. However, the minimization of active APs may excessively degrade the performance of WLANs, where more active APs can usually increase it. In this paper, we propose an extension of the AP aggregation algorithm to ensure the minimum average throughput for each host in the field. This average throughput is estimated using the cost function defined in the algorithm, and the number of active APs is increased when it does not reach the given threshold. We verify the effectiveness of this extension through extensive simulations in two network instances using the WIMNET simulator.