Photoacoustic (PA) imaging is becoming an important tool for various clinical and pre-clinical applications. Acquiring pre-beamformed channel ultrasound data is essential to reconstruct PA images. Accessing these pre-beamformed channel data requires custom hardware to allow parallel beam-forming, and is available for only few research ultrasound platforms. However, post-beamformed radio frequency (RF) data is readily available in real-time and in several clinical and research ultrasound platforms. To broaden the impact of clinical PA imaging, our goal is to devise new PA reconstruction approach based on these post-beamformed RF data. In this paper, we propose to generate PA image by using a single receive focus beamformed RF data. These beamformed RF data are considered as pre-beamformed input data to a synthetic aperture beamforming algorithm, where the focal point per received RF line is a virtual element. The image resolution is determined by the fixed focusing depth as well as the aperture size used in fixed focusing. In addition, the signal-to-noise (SNR) improvement is expected because beamforming is performed twice with different noise distribution. The performance of the proposed method is analyzed through simulation, the practical feasibility is validated experimentally. The results indicate that the post-beamformed RF data has potential to be re-beamformed to a PA image using the proposed synthetic aperture beamformer.