Hi,
As far as I know, you have to read the REG Dicom file's tags, then create an ITK transform file. You could call "plastimatch register ..." to get a dummy ITK transform file. I pasted an example of a 6DOF rigid registration transform file below my signature. I hope it helps.
Best regards,
Dylan
------------------------------------------------------------------------
#Insight Transform File V1.0
#Transform 0
Transform: VersorRigid3DTransform_double_3_3
Parameters: 0.0452169866 -0.0380366993 -0.0333445824 5.8242016864 20.9197180105 73.0415571919
FixedParameters: 0 0 0