A \(re)newcommand that does not care if a command is already defined

There is \providecommand that defines a macro if it is not yet defined. Together with \renewcommand you have your "\extranewcommand":

\providecommand{\foo}{}
\renewcommand{\foo}[1]{bar: #1}

Or you can switch to the plain TeX primitives (other syntax!), e.g.:

\long\def\foo#1{bar: #1}

(\renewcommand and friends without star use \long\def and if the star form is given, \def without \long is used. Also the parameters are specified differently.)

\declarecommand

A definition of the "requested" \declarecommand that calls \providecommand and \renewcommand together:

\documentclass{article}

\makeatletter
\newcommand*{\declarecommand}{%
  \@star@or@long\declare@command
}
\newcommand*{\declare@command}[1]{%
  \provide@command{#1}{}%
  % \let#1\@empty % would be more efficient, but without error checking
  \renew@command{#1}%
}
\makeatother

\begin{document}

\declarecommand*{\foo}{\typeout{foo 1: \meaning\foo}}
\foo
\declarecommand{\foo}{\typeout{foo 2: \meaning\foo}}
\foo
\declarecommand*{\foo}[1]{\typeout{foo #1: \meaning\foo}}
\foo{3}

\end{document}

Result:

foo 1: macro:->\typeout {foo 1: \meaning \foo }
foo 2: \long macro:->\typeout {foo 2: \meaning \foo }
foo 3: macro:#1->\typeout {foo #1: \meaning \foo }

It's not as crafty as Heiko's answer but another option would be

\ifdefined\foo
\renewcommand{\foo}[1]{bar: #1}
\else
\newcommand{\foo}[1]{bar: #1}
\fi

Note that \ifdefined is part of the e-TeX extension and as such part of any modern LaTeX compiler. No packages required.