My matrix-generating-skills are not the most elegant, but is this what you are looking for?
require(xtable)
df <- data.frame(col1=rnorm(10),col2=runif(10,1,3))
mdat <- matrix(c(rep(0,3), 0, 5, 0, rep(0, (7*3)), 0, 5, 0),
nrow = 10, ncol=3, byrow=TRUE)
xtable(df,digits=mdat)
% latex table generated in R 2.15.1 by xtable 1.7-0 package
% Fri Jan 18 15:52:41 2013
\begin{table}[ht]
\begin{center}
\begin{tabular}{rrr}
\hline
& col1 & col2 \\
\hline
1 & -0 & 3 \\
2 & 1.16203 & 1 \\
3 & 1 & 2 \\
4 & 0 & 2 \\
5 & 0 & 2 \\
6 & -1 & 2 \\
7 & 1 & 2 \\
8 & 1 & 2 \\
9 & -1 & 2 \\
10 & 0.63731 & 2 \\
\hline
\end{tabular}
\end{center}
\end{table}
Or the whole row with,
require(xtable)
df <- data.frame(col1=rnorm(10),col2=runif(10,1,3))
mdat <- matrix(c(rep(0,3),rep(5,3), rep(0, (7*3)), rep(5,3)),
nrow = 10, ncol=3, byrow=TRUE)
xtable(df,digits=mdat)
% latex table generated in R 2.15.1 by xtable 1.7-0 package
% Fri Jan 18 16:00:12 2013
\begin{table}[ht]
\begin{center}
\begin{tabular}{rrr}
\hline
& col1 & col2 \\
\hline
1 & -0 & 1 \\
2 & 1.36652 & 2.10159 \\
3 & 0 & 2 \\
4 & -2 & 2 \\
5 & -1 & 2 \\
6 & 0 & 2 \\
7 & -0 & 2 \\
8 & -0 & 1 \\
9 & -1 & 2 \\
10 & -0.44182 & 2.09663 \\
\hline
\end{tabular}
\end{center}
\end{table}