L~this pa per we introduce a methodology for approximately characterizing the superpo sition process of J.V~2 arbitrary discrete-time Markov Renewal Processes (MP2). The superposition process is also a~..,,1RP with a state space that grows exponentially with N. We introduce the arbitrary ani off traffic source model as a special case of the general~. For this special case, we devise an iterative algorithm which can be used to characterize the superposition process in a compact form. Subsequently, a queueing model for a FIFO finite-buffer multiplexer with arbitrary ani off input sources is analyzed. We provide extensive numerical and validation results for the algorithms introduced in the paper. We also study the effect of some of the statistical properties of ani off input sources on the multiplexer's performance.