It would be more interesting to have something like luacalc [1] available

in context. As it is only another kind of table one could insert the data

with a syntax like this

\startspreadsheet

  \startrow

    \startcell 3 \stopcell

    \startcell 5 \stopcell

    \startcell = A1 + A3 \stopcell

  \stoprow

\stopspreadsheet

and the final result is printed with a natural table.

[1] http://luacalc.sourceforge.net/

Wolfgang