siunitx align numrange

The following isn't a direct solution to the problem you're trying to solve. Instead, I suggest you consider pursuing a different solution. Why? Personally, I think that if one tries to present an entire range of numbers in a single column, too much numerical information is being crammed into a confined space. In order to give your readers a chance to really parse and absorb the data you're presenting, it's probably preferable for you to list the minima and maxima in separate columns -- joined, of course, by an overall header that says "Range". The following MWE illustrates what I'm trying to express.

The MWE also uses the \toprule, \midrule, \cmidrule, and \bottomrule macros of the booktabs package to generate well-spaced "rules" (horizontal lines) of varying thickness. These make a table immediately much more professional-looking, compared to what's achievable with \hline and \cline (the basic LaTeX rule-drawing commands).

\documentclass{book}
\usepackage[T1]{fontenc}
\usepackage{siunitx,booktabs} 
% booktabs package for well-spaced horizontal rules

\begin{document} 
\begin{table}
\caption{All measured and their estimated outcome variables during 30 minutes of chair exercises}
\label{table:outcomes}

\medskip\centering
\sisetup{table-format=2.1}
\begin{tabular}{l SSSS S[table-format=3.1]}
\toprule
& {Mean}&{SD} & {Median}& \multicolumn{2}{c}{Range}\\
\cmidrule{5-6}
& & & & {Lower} & {Upper}\\
\midrule
\%vo2max (mL/min) & 66.0 & 16.5 & 66.9 & 30.3 & 115.8\\
&&& 54.6 & 30.3 &  77.7\\
&&& 69.9 & 43.8 & 115.8 \\
\bottomrule
\end{tabular}
\end{table}
\end{document}

I had the same question and I tryed a lot of ways, so there is the way how I solve it:

1. Split the last column in 3 different columns

2. The last 3 columns must be in the format S, c and S

3. Reduce the gap between the columns to 1ex

4. In the last 3 columns write your ranges: the lower value in one column, your range-phrase in other column and the upper value in the last column

Your MWE:

\documentclass{book}
\usepackage[T1]{fontenc}
\usepackage{siunitx} 
\begin{document}
\begin{table}
\caption{All measured and their estimated outcome variables during 30 minutes of chair exercises}
\label{table:outcomes}
\sisetup{
range-phrase = --
}
\begin{tabular}{%
l
c
c
c
S[table-format=3.1, table-text-alignment=center]    %<-- Columns splited
@{\hspace{1ex}}                                     %<-- Column separation
c                                                   %<-- Columns splited
@{\hspace{1ex}}                                     %<-- Column separation
S[table-format=3.1, table-text-alignment=center]    %<-- Columns splited
}
& Mean & SD & Median & \multicolumn{3}{c}{Range}\\ %I just used multicolumn for the splited columns
\hline
\%vo2max \si{\mL/\minute} & 66.0 & 16.5 & 66.9 & 30.3 & -- & 115.8 \\ %<- I use the \si option for the units
                          &      &      & 54.6 & 30.3 & -- & 77.7  \\ %<- Ranges splited and separated by your own range-phrase
                          &      &      & 69.9 & 43.8 & -- & 115.8 \\
\hline
\end{tabular}
\end{table}
\end{document}