As Vladimir Kalitvianski says, it is very much possible in a microscopic sense. Highly ionized plasma is a soup of charged particles bumping into each other via Coulombic interaction and thus radiating braking radiation all the time. However, once this radiation is created, it does not take long to encounter another charged particle which scatters it in a different direction. As a result, many plasmas can be understood as optically thick below the plasma oscillation frequency - immediately absorbing and reradiating light with a thermal spectrum in random directions.
I.e., plasmas are full of radiation of every frequency chaotically scattering around the place, it is just that ordered electromagnetic waves can arise only as collective plasma oscillations (plasmons) with specific dispersion relations.