  We present the first spectroscopic ALMA follow-up for a pilot sample of nine
Radio-Selected NIRdark galaxies in the COSMOS field. These sources were
initially selected as radio-detected sources (S(3GHz)>12.65 uJy), lacking an
optical/NIR counterpart in the COSMOS2015 catalog (Ks>24.7 mag), with just
three of them subsequently detected in the deeper COSMOS2020. Several studies
highlighted how this selection could provide a population of highly
dust-obscured, massive, and star-bursting galaxies. With these new ALMA
observations, we assess the spectroscopic redshifts of this pilot sample of
sources and improve the quality of the physical properties estimated through
SED-fitting. Moreover, we measure the quantity of molecular gas present inside
these galaxies and forecast their potential evolutionary path, finding that the
RS-NIRdark galaxies could represent a likely population of high-z progenitors
of the massive and passive galaxies discovered at z~3. Finally, we present some
initial constraints on the kinematics of the ISM within the analyzed galaxies,
reporting a high fraction (~55%) of double-peaked lines that can be interpreted
as the signature of a rotating structure in our targets or with the presence of
major mergers in our sample. Our results presented in this paper showcase the
scientific potential of (sub)mm observations for this elusive population of
galaxies and highlight the potential contribution of these sources in the
evolution of the massive and passive galaxies at high-z.
