Summenbildung in einer Tabelle

\documentclass{article}
\usepackage[]{longtable}
\usepackage[left=2.00cm, right=2.00cm, top=2.00cm, bottom=2.00cm,includeheadfoot]{geometry}
\usepackage{array}
\newcolumntype{L}[1]{>{\raggedright\arraybackslash}p{#1}} 
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}} 
\usepackage{collcell}
\usepackage{xparse}

\ExplSyntaxOn
\int_new:N \g__colsum_call_int
\int_new:N \g__colsum_start_int
\newcolumntype\columnsum[2]
  {
    >{ \__colsum_set_name:n { #1 } \collectcell \__colsum_calc:n }
    #2
    <{ \endcollectcell }
  }
\cs_new_protected:Npn \__colsum_set_name:n #1
  {
    \tl_set:Nn \l__colsum_name_tl { g__colsum_var_#1_fp }
    \fp_if_exist:cF { \l__colsum_name_tl }
      {
        \fp_new:c { \l__colsum_name_tl }
      }
  }
\cs_new_protected:Npn \__colsum_calc:n #1
  {
    #1
    \int_gincr:N \g__colsum_call_int
    \tl_if_in:nnF { #1 } { \colsumexcept }
      {
        \tl_if_blank:nF { #1 }
          {
            \int_compare:nNnT { \g__colsum_call_int } > { \g__colsum_start_int }
              {
                \fp_gset:cn { \l__colsum_name_tl }
                  { \fp_use:c { \l__colsum_name_tl } + #1 }
              }
          }
      }
  }
\NewDocumentCommand \colsumstart { m }
  {
    \int_gset:Nn \g__colsum_start_int { #1 }
    \int_gzero:N \g__colsum_call_int
  }
\NewExpandableDocumentCommand \colsumvar { o m }
  {
    \IfNoValueTF { #1 }
      { \fp_use:c { g__colsum_var_#2_fp } }
      { \fp_eval:n { round ( \fp_use:c { g__colsum_var_#2_fp } , #1 ) } }
  }
\NewDocumentCommand \colsumvarset { m m }
  {
    \fp_if_exist:cF { g__colsum_var_#1_fp }
      { \fp_new:c { g__colsum_var_#1_fp } }
    \fp_gset:cn { g__colsum_var_#1_fp } { #2 }
  }
\cs_set:Npn \colsumexcept {}
\ExplSyntaxOff

\begin{document}
% we don't really know the row, but only the call at which we want to start. As
% it isn't called in a \multicolumn, we have to care for that
\colsumstart{4}
\begin{longtable}{|L{6cm}|\columnsum{M}{C{2cm}}|C{2cm}|C{1.4cm}|C{1.4cm}|\columnsum{Izz}{C{1.4cm}}|}
    \caption{M}
    \label{tab:mList} \\
    \hline
    \multicolumn{1}{|l|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{3}{c|}
    {\textbf{Trägheitsmomente}}\\
    \textbf{Body} & \textbf{Masse} [kg] & \textbf{Typ} & \textbf{Ixx} [kgm$^2$] &  \textbf{Iyy} [kg*m$^2$] &  \textbf{Izz} [kg*m$^2$] \\
    \hline
    \noalign{\vskip 0.08cm}  
    \hline
    \endfirsthead
    \multicolumn{6}{c}
    {\tablename\ \thetable.\ -- \textbf{z} -  Fortsetzung} \\
    \hline
    \multicolumn{1}{|l|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{3}{c|}{t}\\
    \textbf{Body} & \textbf{Masse} [kg] & \textbf{Typ} & \textbf{Ixx} [kgm$^2$] &  \textbf{Iyy} [kgm$^2$] &  \textbf{Izz} [kgm$^2$] \\
    \hline
    \noalign{\vskip 0.08cm}  
    \hline
    \endhead
    \multicolumn{6}{c}{Tabelle wird fortgesetzt}
    \endfoot
    \endlastfoot
    \$S\_Wk1.\$ & 15601 & 0 & 34322 & 592838 & 592838\\
    \hline
    \$S\_Wk1.\$ & 1 & 0 & 1 & 1 & 1\\
    \hline
    \$S\_Wk1.\$B & 1 & 0 & 1 & 1 & 1\\
    \hline
    \$S\_1T.\$B\_Wks1 & 0 & 0 & 1 & 1 & 1\\
    \hline
    \$S\_1T.\$B\_ & 52 & 0 & 0.6 & 1.1 & 0.8\\
    \hline
    \$S\_1T.\$B\_2 & 52 & 0 & 0.6 & 1.1 & 0.8\\
    \hline
    \textbf{Ergebnis Summe von Masse} &
    % \colsumexcept tells it to not use this cell in the sum.
    \colsumexcept\colsumvar{M} & & & &
    \colsumexcept\colsumvar{Izz}
    \\
    \hline  
\end{longtable}
\end{document}