Hi Joel,
You can include any combination of GHI, DNI, DHI, and POA irradiance data in your weather file in the SAM CSV format. See the format description in the "SAM CSV Format for Solar" topic of SAM's Help system:
samrepo.nrelcloud.org/help/weather_format_sam_csv_solar.htm
If your weather file includes POA data, to use it as input for the Detailed PV model, in a simulation, choose the appropriate "POA" option under Albedo - Sky Diffuse Model - Irradiance Data (Advanced) on the Location and Resource page.
If you use POA irradiance data as input, SAM only uses the POA irradiance data from the weather file. It uses a decomposition model to calculate the DHI and DNI values from the POA data. That is described more in the "Location and Resource" Help topic in the section for the advanced inputs:
samrepo.nrelcloud.org/help/pv_location_and_resource.htm
Best regards,
Paul.