in general the transmitted bits are generated randomly under a full buffer assumption. This means, it is assumed that there is always data to transmit if no traffic model is used. The number of bits corresponds to the currently employed modulation and coding scheme and also depends on the number of active spatial streams (number of transmitted codewords). So the number of transmitted bits you find in the results are just the physical layer throughput per TTI and SNR. If there is no traffic model employed, which is the default situation in our link level simulator, there is no such thing as higher layer packed or a loaded network condition.
In the Vienna LTE-A Link Level Downlink Simulator there are some traffic models implemented that generate traffic according to some service. In this case, there is no more full buffer assumption, because there is not always data to transmit. However, as I said, these traffic models are not very mature. As you suggested you would have to modify and improve these models for your needs. This, of course, means changes in the traffic model and probably also in the scheduler.