documentclass{article}

usepackage{spreadtab}
usepackage{array}
usepackage{etoolbox}
usepackage{tabularx}

newcounter{tabline}

makeatletter
gdefctag#1{@ifundefined{c#1}
{expandafterxdefcsname c#1endcsname{thetabline}}{expandafterxapptocsname c#1endcsname{,thetabline}}}
makeatother

begin{document}

begin{spreadtab}{{tabular}{>{stepcounter{tabline}thetabline}cc}}
 & @ a\ @ctag{1}
  & @ b \
   & @ c\ @ctag{1}
   & @ d\
end{spreadtab}

csname c1endcsname
end{document}

begin{document}

begin{spreadtab}{{tabular}{>{stepcounter{tabline}thetabline}cc}}
 & @ a\ @ctag{1}
  & @ b \
   & @ c\ @ctag{1}
   & @ d\
end{spreadtab}

csname c1endcsname
end{document}