  The extension of the Standard Model lepton sector by three right-handed
Majorana neutrinos (heavy neutral leptons, HNL) with masses up to GeV scale is
considered. While the lightest HNL is the dark matter particle with mass of the
order of 5 keV, the remaining two HNLs ensure standard (active) neutrino mass
generation by means of the see-saw type I mechanism. Two heavy sterile
neutrinos with quasi-degenerate masses up to 5 GeV can induce the deviation of
lepton universality violation parameter in the decays of $\pi^\pm$ and $K^\pm$
mesons from the the Standard Model value. Contours are obtained for the
permissible values of this parameter within the framework of two mixing
scenarios, taking into account the lifetime boundary for heavy neutral lepton
from Big Bang nucleosynthesis in the Universe. When calculating the HNL decay
width in the framework of the model with six Majorana neutrinos, three active
and three heavy, both two-particle and three-particle lepton decays, essential
for masses below the mass of the pion, were taken into account. When
calculating the decay widths, the limiting case known as the "Dirac limit" is
not used. The results based on the explicit form of mixing matrices for three
HNL generations and the diagram technique for Majorana neutrinos, which
explicitly take into account the interference terms for diagrams with identical
mass states, can lead to some differences in lifetime from the results using
the "Dirac limit" and the displacement of the corresponding experimental
exclusion contours of the "mass-mixing" type. For the second mixing scenario, a
mass region of $460~\text{MeV} <M< 485$ MeV has been found that allows
violation of lepton universality in charged kaon decays at the level observed
in the experiment.
