Automated frameworks for approximate accelerator synthesis employ an iterative search-based approach to generate approximate instances of hardware. While offering distinct savings in terms of hardware area and power consumption, approximate circuits are potentially at risk of being infected with hardware Trojans mainly due to the fact that the approximation is typically provided by third-party approximate accelerator synthesis frameworks which utilize components libraries to perform substitutions during the design space exploration phase. In this paper, we propose a threat model that discusses the potential of hardware Trojans insertion during the approximate accelerator synthesis. Moreover, we present MAAS, a framework that exploits a search-based approximate accelerator synthesis technique to demonstrate the applicability of our threat model by hiding Trojans in approximate circuits. The experimental results show that the approximate circuits generated by MAAS containing infected hardware Trojans are slightly larger than the approximate designs and are hard to identify via conventional area and power measurement techniques. To the best of our knowledge, this is the first effort to demonstrate the hardware Trojan insertion in the third-party approximate accelerator synthesis flow.