glossaries: How to customize list of symbols with additional column for units?

I suggest to use a separate unit key for the symbol unit and refer to it with \glsunit. (Alternative: Use the user1...user6 keys for this feature)

In addition, this key must be used in the glossarystyle. I've defined a new style based on the already provided long3col style which displays the symbols in the requested order.

\documentclass{book}                           
\usepackage{siunitx}
\usepackage[acronym,toc]{glossaries}              % use glossaries-package


\setlength{\glsdescwidth}{15cm}

\newglossary[slg]{symbolslist}{syi}{syg}{Symbolslist} % create add. symbolslist


\glsaddkey{unit}{\glsentrytext{\glslabel}}{\glsentryunit}{\GLsentryunit}{\glsunit}{\Glsunit}{\GLSunit}

\makeglossaries                                   % activate glossaries-package


% ==== EXEMPLARY ENTRY FOR SYMBOLS LIST =========================================
    \newglossaryentry{symb:Pi}{name=\ensuremath{\pi},
        description={Geometrical value},
        unit={},
        type=symbolslist}

    \newglossaryentry{height}{name=\ensuremath{h},
        description={Height of tower},
        unit={\si{m}},
        type=symbolslist}

    \newglossaryentry{energyconsump}{name=\ensuremath{P},
        description={Energy consumption},
        unit={\si{kW}},
        type=symbolslist}




% ==== EXEMPLARY ENTRY FOR ACRONYMS LIST ========================================
    \newacronym{VRBD}{VRBD}{Violet-Red-Bile-Glucose-Agar}


% ==== EXEMPLARY ENTRY FOR MAIN GLOSSARY ========================================
    \newglossaryentry{Biofouling}{name=Biofouling,description={Some description}}


\newglossarystyle{symbunitlong}{%
\setglossarystyle{long3col}% base this style on the list style
\renewenvironment{theglossary}{% Change the table type --> 3 columns
  \begin{longtable}{lp{0.6\glsdescwidth}>{\centering\arraybackslash}p{2cm}}}%
  {\end{longtable}}%
%
\renewcommand*{\glossaryheader}{%  Change the table header
  \bfseries Sign & \bfseries Description & \bfseries Unit \\
  \hline
  \endhead}
\renewcommand*{\glossentry}[2]{%  Change the displayed items
\glstarget{##1}{\glossentryname{##1}} %
& \glossentrydesc{##1}% Description
& \glsunit{##1}  \tabularnewline
}
}


\begin{document}

    \glsaddall

    \printglossary[type=\acronymtype,style=long]  % list of acronyms
    \printglossary[type=symbolslist,style=symbunitlong]   % list of symbols
    \printglossary[type=main]                     % main glossary

\end{document}

Just adding to Christian's answer: to use the unit macros inside the \si{unit} command defined by siunitx such as \metre and \kilo\watt used in his MWE, one should add the command \glssetnoexpandfield{unit} before setting any entries.

It is better explained in This Answer.