Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Middle Point
MDATA_SPLINEARC_MIDPOINT
Parameter: End Point
MDATA_SPLINEARC_ENDPOINT
Parameter: First Point
MDATA_SPLINEARC_FIRSTPOINT
Parameter: Center
MDATA_SPLINEARC_CENTER
Parameter: Radius
MDATA_SPLINEARC_RADIUS
Parameter: Angle
MDATA_SPLINEARC_ANGLE
Parameter: First Insert Spline Point Index
MDATA_SPLINEARC_POINTINDEX1
Parameter: Second Insert Spline Point Index
MDATA_SPLINEARC_POINTINDEX2
Parameter: Object First Insert Spline Point Index Belongs To
MDATA_SPLINEARC_OBJECT1
Parameter: Object Second Insert Spline Point Index Belongs To
MDATA_SPLINEARC_OBJECT2