Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Start Angle
FIELD_RADIAL_STARTANGLEParameter: Start Transition
FIELD_RADIAL_STARTBLENDParameter: End Angle
FIELD_RADIAL_ENDANGLEParameter: End Transition
FIELD_RADIAL_ENDBLENDParameter: Iterations
FIELD_RADIAL_ITERATIONSParameter: Offset
FIELD_RADIAL_OFFSETParameter: Axis
FIELD_RADIAL_AXISParameter: Clip to Shape
FIELD_RADIAL_CLIPParameter: Offset
FIELD_RADIAL_OFFSETFIELD