I'm modeling a building for LEED EAc1 compliance, and was wondering how
you are supposed to model existing building HVAC systems if you are
adding square footage to the building.

Let's assume you have a 4-story building with a 2-chiller HVAC plant and
dual temp boxes. If you were to add a 5th floor to the building, what
would your baseline model look like? I know that the 1st four floors
would use the existing HVAC system, but would your new 5th floor be
treated as a new 15,000 square foot area with packaged RTUs, or would
you add a new chiller to the existing HVAC chilled water plant, and have
that chiller meet all the requirements of App G, and have shutoff VAV
boxes for the 5th floor in lieu of dual temp boxes?

For reference, under our proposed design, we are doing just that:
adding a frictionless chiller to the plant to make up the extra load.

James Hansen, PE, LEED AP