Several access technologies are expected to coexist and share the same spectrum portion for next-generation networks. However, to date, few tools exist in the literature when it comes to evaluating protocol performance in scenarios implying the coexistence of heterogeneous access technologies. We report herein, an extension of an open-source simulation framework for OMNeT++ (INET) to simulate interference impact on the coexistence of IEEE 802.11 and IEEE 802.15.4 in the 2.4GHz ISM bands. We extended the interference module of INET to take into account inter-technology and inter-channel interferences.