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I found your question because I have the same one. In Energy Plus, I was able to add a relief fan as a component on the AirLoopHVAC:OutdoorAirSystem, just by modifying the AirloopHVAC:OutdoorAirSystem:EquipmentList and adding the Relief fan object. I started with the expanded IDF from the example file "HVACTemplate-5ZoneVAVWaterCooled.idf". I checked the reports on fan power and the node flow exiting the fan and it seems to work. Here are the objects I modified/added:

 AirLoopHVAC:OutdoorAirSystem:EquipmentList,
   VAV Sys 1 OA System Equipment,                           !- Name
   OutdoorAir:Mixer,                                        !- Component Object Type
   VAV Sys 1 OA Mixing Box,                                 !- Component Name
   Fan:VariableVolume,                                        !- Component Object Type
   VAV Sys 1 Relief Fan;                                 !- Component Name

 Fan:VariableVolume,
   VAV Sys 1 Relief Fan,                                    !- Name
   FanAvailSched,                                           !- Availability Schedule Name
   0.7,                                                     !- Fan Efficiency
   600,                                                     !- Pressure Rise {Pa}
   autosize,                                                !- Maximum Flow Rate {m3/s}
   Fraction,                                                !- Fan Power Minimum Flow Rate Input Method
   0.25,                                                    !- Fan Power Minimum Flow Fraction
   ,                                                        !- Fan Power Minimum Air Flow Rate {m3/s}
   0.9,                                                     !- Motor Efficiency
   1,                                                       !- Motor in Airstream Fraction
   0.35071223,                                              !- Fan Power Coefficient 1
   0.30850535,                                              !- Fan Power Coefficient 2
   -0.54137364,                                             !- Fan Power Coefficient 3
   0.87198823,                                              !- Fan Power Coefficient 4
   0,                                                       !- Fan Power Coefficient 5
   VAV Sys 1 Relief Air Outlet,                           !- Air Inlet Node Name
   VAV Sys 1 Relief Fan Outlet;                             !- Air Outlet Node Name

I found your question because I have the same one. In Energy Plus, I was able to add a relief fan as a component on the AirLoopHVAC:OutdoorAirSystem, just by modifying the AirloopHVAC:OutdoorAirSystem:EquipmentList and adding the Relief fan object. I started with the expanded IDF from the example file "HVACTemplate-5ZoneVAVWaterCooled.idf". I checked the reports on hourly output variables for the fan power and the node flow exiting the fan and it seems to work. Here are the objects I modified/added:

 AirLoopHVAC:OutdoorAirSystem:EquipmentList,
   VAV Sys 1 OA System Equipment,                           !- Name
   OutdoorAir:Mixer,                                        !- Component Object Type
   VAV Sys 1 OA Mixing Box,                                 !- Component Name
   Fan:VariableVolume,                                        !- Component Object Type
   VAV Sys 1 Relief Fan;                                 !- Component Name

 Fan:VariableVolume,
   VAV Sys 1 Relief Fan,                                    !- Name
   FanAvailSched,                                           !- Availability Schedule Name
   0.7,                                                     !- Fan Efficiency
   600,                                                     !- Pressure Rise {Pa}
   autosize,                                                !- Maximum Flow Rate {m3/s}
   Fraction,                                                !- Fan Power Minimum Flow Rate Input Method
   0.25,                                                    !- Fan Power Minimum Flow Fraction
   ,                                                        !- Fan Power Minimum Air Flow Rate {m3/s}
   0.9,                                                     !- Motor Efficiency
   1,                                                       !- Motor in Airstream Fraction
   0.35071223,                                              !- Fan Power Coefficient 1
   0.30850535,                                              !- Fan Power Coefficient 2
   -0.54137364,                                             !- Fan Power Coefficient 3
   0.87198823,                                              !- Fan Power Coefficient 4
   0,                                                       !- Fan Power Coefficient 5
   VAV Sys 1 Relief Air Outlet,                           !- Air Inlet Node Name
   VAV Sys 1 Relief Fan Outlet;                             !- Air Outlet Node Name