Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Radius
MDATA_SPLINESKETCH_RADIUSParameter: Stroke Smoothing
MDATA_SPLINESKETCH_SMOOTHINGParameter: Blend
MDATA_SPLINESKETCH_BLENDParameter: Create New Spline
MDATA_SPLINESKETCH_NEWSPLINE