Hi Pão,
If you have measured GHI and DNI data, then you can put those in the GHI and DNI columns of your weather file. If you inserting your data into a file from the NSRDB (or any other source), you could rename the original GHI and DNI column headings so that SAM's weather file reader can correctly interpret the data. For example you could change the column heading "GHI" to "NSRDB GHI" so the reader will ignore that column.
Also, in SAM, on the Location and Resource page, be sure to change the Weather File Irradiance Data option under Albedo - Sky Diffuse Model - Irradiance Data (Advanced) to DNI and GHI.
Best regards,
Paul.