For IS wind load, I don't believe there is any option for open frame wind load generation, only auto-generation of wind loads per rigid diaphragm extents or manually assigning wind coefficients to area assignments. In addition to assigning wind coefficients to walls in your model, you can also draw dummy/null areas to assign wind coef. using area property "NONE".
Use SAP2000 documentation and go under manuals>Lateral load manual and you'll find more specifics on how IS 875 is handled
