Dear James,
In the current version of SAM (SAM 2015.6.30 r2), it is not possible to use POA irradiance as input to SAM's detailed photovoltaic model. However, we are working on that capability now, and it will be part of the next version of SAM, which we plan to release late this calendar year or early next.
For now, you would have to convert the POA data to its components. I would recommend using the SAM CSV format rather than the TMY3 format to do that. The SAM CSV format is simpler and only stores the data that SAM requires. The format is described in SAM's Help system, under Weather Data, Weather File Formats.
Best regards,
Paul.