Welcome to the OptoForums! You are correct, that slave part, built into the memory map, is not included in SoftPAC.
However, our [U][B]Modbus Integration Kit (click here)[/B][/U] includes logic (one chart and one subroutine) to turn your PAC into a Modbus Slave or Master (the rest of the charts/subs are for that) and this strategy (or just the pieces of it you need) will work on SoftPAC or PAC Sim as well as a hardware controller.
What devices are you connecting together? What are you building?