For a rather broad class of dynamical systems subject to mixed fermionic
first and second class constraints or infinitely reducible first class
constraints (IR1C), a manifestly covariant scheme of supplementation of IR1C to
irreducible ones is proposed. For a model with IR1C only, an application of the
scheme leads to a system with covariantly splitted and irreducible first and
second class constraints. Modified Lagrangian formulations for the
Green--Schwarz superstring, Casalbuoni--Brink--Schwarz superparticle and Siegel
superparticle, which reproduce the supplementation scheme, are suggested
