As the Wi-Fi technology goes through its sixth generation (Wi-Fi 6), there is a growing consensus on the need to support security and coordination functions at the Physical (PHY) layer, beyond traditional functions such as frame detection and rate adaptation. In contrast to the costly approach of extending the PHY-layer header to support new functions (e.g., Target Wake Time field in 802.11ax), we propose to turn a specific part of the frame preamble into a data field while maintaining its primary functions. Specifically, in this paper, we develop a scheme called extensible preamble modulation (ePMod) for the MIMO-based 802.11ac protocol. For each frame, eP-Mod can embed up to 20 bits into the 802.11ac preamble under 1 × 2 or 2 × 1 MIMO transmission modes to support the operations of a given PHY-layer function. The proposed scheme enables several promising PHY-layer services, such as PHY-layer encryption and channel/device authentication, PHYlayer signaling, etc. At the same time, it allows legacy (eP-Modunaware) devices to continue to process the received preamble as normal by guaranteeing that our proposed preamble waveforms satisfy the structural properties of a standardized preamble. Through numerical analysis, extensive simulations, and hardware experiments, we validate the practicality and reliability of ePMod.