You can use the Convert Interleave tool to do this:
http://www.harrisgeospati...erData.html#Convert2 If the data is from a known sensor, you can use the Radiometric Calibration tool and select 'FLAASH Settings'. It will convert the data to the expected units and interleave for FLAASH:
http://www.harrisgeospati...tricCalibration.html