Exploiting the increasing ubiquitous deployment of sensor networks, the paper presents a system called WiBee that utilizes ZigBee sensors to build real-time WiFi radio maps. The design of WiBee is motivated by the observation that a ZigBee radio can sense WiFi frame transmissions although it cannot decode WiFi frames. A sensor passively listens on the wireless channel and estimates the RSS of a WiFi AP at its location. The design of WiBee faces three unique challenges. First, multiple APs may transmit frames concurrently and frame collisions may happen. Second, because of severe resource constraints, a sensor cannot sample the channel at arbitrarily high frequency and hence some frame transmissions may not be sampled. Third, sensor nodes are usually not time synchronized and the on-board clock is inaccurate. To address these challenges, we propose a novel gateway-assisted approach to estimating WiFi RSS values at ZigBee sensors. A light-weight algorithm is designed for identifying the RSS values corresponding to a given AP. It searches the sequence of ZigBee RSS samples for an AP signature sequence. An optimization technique is proposed to address issues of clock drift and time asynchronization. Our extensive experiments on a testbed show that WiBee can achieve low estimation error, short delay and small computation overhead.