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...
