Welcome to the OptoForums! You are correct, that slave part, built into the memory map, is not included in SoftPAC.
However, our Modbus Integration Kit (click here) 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?