We show that in the supersymmetry framework described by a Poincar\'{e} superalgebra with tensorial central charges the role of generalized superconformal symmetry which contains all these central charges is played by \(OSp(1|2^{k})\), where k=3 for D=4. Following [1,2] we describe the free supertwistor model for \(OSp(1|8)\). It appears that in such a scheme the tensorial central charges satisfy additional relations and the model describes the tower of supersymmetric massless states with an arbitrary (integer and half--integer) helicity spectrum.