Dear Swilson,
You have two options to calculate the problem:
1-utilize other modeling tools that H2S is available to mix with other components.
2-if you must use the PHAST, you can substitute the H2S with another component(s) which its molecular weight is around 34.08g/mol (say Ethane and a little Propane) and then you have to find the related dispersion concentration in PPM according to H2S ERPGs same as follows:
ERPG-1=0.1ppm
ERPG-2=30ppm
ERPG-3=100ppm
Regards