yes that is possible. When starting to develop the Vienna 5G LLS, we structured it such that this is possible. However, as we currently don’t see D2D communications as a hot topic for link level, we do not plan to implement this feature in the near future.
Form the main.m structure you can see how the D2D part would fit in. However, in total many changes are required for this feature. The main ones probably being the modification of the scheduler, implementation of the corresponding transmitter and receiver functions within the UE class and saving of the results.