Custom theorem numbering

An easy way is

\documentclass{article}
%\usepackage{amsthm} %% uncomment to see the difference
\newtheorem{innercustomthm}{Theorem}
\newenvironment{customthm}[1]
  {\renewcommand\theinnercustomthm{#1}\innercustomthm}
  {\endinnercustomthm}

\begin{document}

\begin{customthm}{8}\label{eight}
Every theorem must be numbered by hand.
\end{customthm}

Here is a reference to theorem~\ref{eight}.
\end{document}

You can also use the optional argument for attribution:

\begin{customthm}{99}[Somebody]\label{ninetynine}
Statement.
\end{customthm}

---

A more generic interface for defining several of these environments; this doesn't respect theorem styles; it could be adapted, though.

\documentclass{article}
\usepackage{amsthm}

\newtheorem{innercustomgeneric}{\customgenericname}
\providecommand{\customgenericname}{}
\newcommand{\newcustomtheorem}[2]{%
  \newenvironment{#1}[1]
  {%
   \renewcommand\customgenericname{#2}%
   \renewcommand\theinnercustomgeneric{##1}%
   \innercustomgeneric
  }
  {\endinnercustomgeneric}
}

\newcustomtheorem{customthm}{Theorem}
\newcustomtheorem{customlemma}{Lemma}

\begin{document}

\begin{customthm}{8}\label{eight}
Every theorem must be numbered by hand.
\end{customthm}

Here is a reference to theorem~\ref{eight} and
one to the important lemma~\ref{life-universe-everything}

\begin{customlemma}{42}\label{life-universe-everything}
This lemma explains everything.
\end{customlemma}


\end{document}

You can just temporarily reset your theorem counter:

\documentclass{article}
\usepackage{amsthm}


\newtheorem{Theorem}{Theorem}



\makeatletter

\newenvironment{customTheorem}[1]
  {\count@\c@Theorem
   \global\c@Theorem#1 %
    \global\advance\c@Theorem\m@ne
   \Theorem}
  {\endTheorem
   \global\c@Theorem\count@}

\makeatother


\begin{document}


\begin{Theorem}
Dummy text
\end{Theorem}

\begin{customTheorem}{99}
Dummy text
\end{customTheorem}

\begin{Theorem}
Dummy text
\end{Theorem}


\end{document}