Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Radius
PRIM_CAPSULE_RADIUSParameter: Height
PRIM_CAPSULE_HEIGHTParameter: Height Segments
PRIM_CAPSULE_HSUBParameter: Cap Segments
PRIM_CAPSULE_FSUBParameter: Rotation Segments
PRIM_CAPSULE_SEGParameter: Switch to Updated Capsule
PRIM_CAPSULE_SWITCH_TO_UPDATED