How to add unnumbered sub and subsubsections to toc and headers?

Question

I'd like to add unnumbered subsections and subsubsections to my toc and my headers using addsec. How could I do that ? This is my MWE:

\documentclass[fontsize=11pt,%
               twoside=semi,%
               headings=small,%
               chapterprefix=true,%
               listof=flat]%
{scrbook}
\usepackage[utf8]{inputenc}% can be removed with an uptodate TeX distribution
\usepackage[T1]{fontenc}
\usepackage [french]{babel}
\usepackage{fontspec}
\setmainfont{Didot}
\usepackage{lipsum}
\usepackage[
  paperheight=21.5cm, paperwidth=15cm,
  left=2cm, right=2cm, top=1.5cm, bottom=2.5cm,
  bindingoffset=7mm,
  includehead,includefoot
  ]{geometry}


%%%%%%%%%%%%%%%%%%%%%%%%%%numéros page%%%%%%%%%%%%
\usepackage[automark]{scrlayer-scrpage}% See scrguien.pdf, chapter 5
\ihead{\headmark}% running head at the inner edge of the page head
\ohead*{\pagemark}% page number at the outer edge of the page head (plain pages too)
\ofoot*{}% no page number at the outer edge of the page footer (plain pages too)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\addtokomafont{pageheadfoot}{\scshape\small}%changed to small caps in headers
\addtokomafont{disposition}{\normalfont}
\addtokomafont{part}{\bfseries\scshape\LARGE}
\addtokomafont{chapter}{\bfseries\scshape\LARGE}
\addtokomafont{section}{\bfseries\scshape\large}
\addtokomafont{subsection}{\bfseries\itshape}
\addtokomafont{subsubsection}{\bfseries\itshape}
\addtokomafont{paragraph}{\scshape\small}
\addtokomafont{subparagraph}{\scshape\small}
%\let\bfseries\mdseries



\renewcommand*{\chapterformat}{%
  \mbox{\chapappifchapterprefix{\nobreakspace}\thechapter%\autodot
  \IfUsePrefixLine{}{\enskip}}%
}




\begin{document}

\tableofcontents

\mainmatter
\addchap{A chapter}
\lipsum
\addsec{A section}
\lipsum
\end{document}

Answer 1

KOMA-Script version 3.27 has introduced an option nonumber=true for all section levels:

\documentclass[
  headings=optiontoheadandtoc% activates the advanced functionality of the optional argument of the sectioning commands
]{scrbook}
\usepackage{blindtext}
\begin{document}
\tableofcontents
\chapter[nonumber=true]{Chapter without number}
\section[nonumber=true]{Section without number}
\subsection[nonumber=true]{Subsection without number}
\Blindtext[10]
\blinddocument
\end{document}

Result:

Answer 2

Unnumbered (sub)subsections can be added to the toc with \addxcontentsline{toc}{subsection}{A Subsection}.

To make subsubsections visible there, also add \setcounter{tocdepth}{4}

In principle one could also add them to the header, but where should they be, the header is already occupied by the section and chapter for odd and even pages. There is no third or forth type of page for them.

\documentclass[fontsize=11pt,%
               twoside=semi,%
               headings=small,%
               chapterprefix=true,%
               listof=flat]%
{scrbook}
\usepackage[utf8]{inputenc}% can be removed with an uptodate TeX distribution
\usepackage[T1]{fontenc}
\usepackage [french]{babel}
%\usepackage{fontspec}
%\setmainfont{Didot}
\usepackage{lipsum}
\usepackage[
  paperheight=21.5cm, paperwidth=15cm,
  left=2cm, right=2cm, top=1.5cm, bottom=2.5cm,
  bindingoffset=7mm,
  includehead,includefoot
  ]{geometry}


%%%%%%%%%%%%%%%%%%%%%%%%%%numéros page%%%%%%%%%%%%
\usepackage[automark]{scrlayer-scrpage}% See scrguien.pdf, chapter 5
\ihead{\headmark}% running head at the inner edge of the page head
\ohead*{\pagemark}% page number at the outer edge of the page head (plain pages too)
\ofoot*{}% no page number at the outer edge of the page footer (plain pages too)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\addtokomafont{pageheadfoot}{\scshape\small}%changed to small caps in headers
\addtokomafont{disposition}{\normalfont}
\addtokomafont{part}{\bfseries\scshape\LARGE}
\addtokomafont{chapter}{\bfseries\scshape\LARGE}
\addtokomafont{section}{\bfseries\scshape\large}
\addtokomafont{subsection}{\bfseries\itshape}
\addtokomafont{subsubsection}{\bfseries\itshape}
\addtokomafont{paragraph}{\scshape\small}
\addtokomafont{subparagraph}{\scshape\small}
%\let\bfseries\mdseries



\renewcommand*{\chapterformat}{%
  \mbox{\chapappifchapterprefix{\nobreakspace}\thechapter%\autodot
  \IfUsePrefixLine{}{\enskip}}%
}

\setcounter{secnumdepth}{4}
\setcounter{tocdepth}{4}


\begin{document}

\tableofcontents

\mainmatter
\addchap{A chapter}
\lipsum
\addsec{A section}
\lipsum


\subsection*{A subsection}
\addxcontentsline{toc}{subsection}{A Subsection}
\subsection*{A subsubsection}
\addxcontentsline{toc}{subsubsection}{A subsubsection}

\end{document}