This is the memoir way:

\newcommand{\extendedsec}[1]{% \makebox[0pt][l]{\parbox[t]{\textwidth}{\Large\bfseries\raggedright#1}}} \setsecheadstyle{\extendedsec} 

Change \textwidth into what suits you best. I wouldn't fill the whole line.

This is the memoir way:

\newcommand{\extendedsec}[1]{\noindent \makebox[0pt][l]{\parbox[t]{\textwidth}{\Large\bfseries\raggedright#1}}} \setsecheadstyle{\extendedsec} 

Change \textwidth into what suits you best. I wouldn't fill the whole line.

If you want to fill the whole line, then

\newcommand{\extendedsec}[1]{\noindent \makebox[0pt][l]{\parbox[t]% {\dimexpr\textwidth+\marginparsep+\marginparwidth\relax}% {\Large\bfseries\raggedright#1}}} 

A similar trick is described on page 106 of the manual for memoir