Hi Matthew,
There is no need to make a correction.
The hourly irradiance data in the NSRDB represents the solar irradiance in W/m2 available at the midpoint of the hour.
SAM's concentrating solar power (CSP) models assume that the hourly DNI value represents the average energy available over the hour, and calculate sun angles at the midpoint of the hour.
For hourly solar resource data, the two assumptions are equivalent.
Best regards,
Paul.