I think you are doing it right. When you create a .SIT file you are just setting the place downloading just monthly meteo data, see image 1. So you get GHI, DHI, Tamb, Wind velocity and RHumidity.
So PVsyst generates a synthetic meteo file hour by hour in order to calculate the simulation. See image below
I do not know how can Pvsyst simulate the system (mainly losses) if no hourly calculations are performed. How did you get results without creating a .SIT file?
As far as I know, the TMY is supposed to provide the "same data" (different format) as the .MET file. So the creation of a .MET form a TMY is just fomat conversion.
To sum up, if you have a TMY file you should import it, create the .SIT and the .MET files after that you should run the simulation selecting the meteo data from .MET.