Additional references in BibTeX

Link.


There are two solutions possible:

1. Patch the \AtEndEnvironment{thebibliography} as follows:

\AtEndEnvironment{thebibliography}{
% all your extra bibitems go here
\bibitem{extra1}A. Uthor, Some Extra Paper, vol 123, p.2--3 
}

Do not forget to put \usepackage{etoolbox} somewhere in the preamble. MWE:

\documentclass{article}

\usepackage{etoolbox}

\begin{document}

\AtEndEnvironment{thebibliography}{
% all your extra bibitems go here
\bibitem{extra1}A. Uthor, Some Extra Paper, vol 123, p.2--3 
}

Cite from above \cite{extra1}.
Cite from bib-file \cite{bookWhatever}.

\bibliographystyle{plain}
\bibliography{/path/to/your.bib_file} 

\end{document}

2. Create a "local" bib file.

Basically, the same trick, as has been suggested already by R. Schumacher.
But, if you prefer to keep everything in one .tex file, just do like in the following MWE:

\begin{filecontents}{mylocalbib.bib}
@article{extra1,
  title={Title},
  author={A. Uthor},
  journal={PRL},
  volume={12},
  pages={123--124},
  year={1999},
}
\end{filecontents}

\documentclass{article}

\begin{document}
\cite{extra1}

\bibliographystyle{plain}
\bibliography{/path/to/your.bib_file, mylocalbib} % here both your "global" and "local" bib-files are mentioned

\end{document}