  Spin observables of a binary reaction $1+2\to 3+4$ are discussed at the
threshold in general form using P-parity and angular momentum conservation.
General formulae for polarization transfer, spin-spin correlation parameters in
the initial and final states and induced tensor polarization are derived for
arbitrary spins of participating particles. This formalism is worked out in
detail for the $NN\to Y\Theta^+$ reaction. Assuming that the spin of the
pentaquark $\Theta^+$ takes the values 1/2, 3/2 and 5/2, whereas the spin of
the hyperon $Y$ equals 1/2, explicite formulae are obtained for the observables
in terms of few non-vanishing at the threshold spin amplitudes separately for
the spin-singlet and spin-triplet initial NN states. In case of all particles
in the $NN\to Y\Theta^+$ reaction have the spin-1/2 a full spin-structure for
totally polarized cross sections is derived. Some of the obtained spin
observables strongly depend on the intrinsic P-parity of the $\Theta^+$ and the
total isospin of the reaction. Therefore, a measurement of these observables
allows one to determine the P-parity of the pentaquark $\Theta^+$ in a model
independent way for any spin of the $\Theta^+$.
