In this paper, we present a hybrid Visible Light Communication (VLC) and Radio Frequency (RF) based Non-Orthogonal Multiple Access (NOMA) system to extend the coverage of VLC and support multi-user communications at the same time. In the proposed system, all users are divided into multiple user pairs, where the near user who can receive the visible light signal directly would help forward the information to its paired far user through RF transmission. To achieve that, NOMA is adopted during the VLC phase so the near user is able to receive not only its own but also the information for its paired user. Aiming at minimizing the outage probability of system, we formulate the optimal user pairing problem and convert it into the one of finding the maximum weighted matching in a bipartite graph. Kuhn-Munkres (KM) algorithm is utilized to solve the problem. Simulation results demonstrate that the system performance is markedly improved with our proposed scheme.