\documentclass[a4paper]{article}
\usepackage[numbers,sort&compress]{natbib}
\makeatletter
\newcounter{X@bibstep}
\setcounter{X@bibstep}{1000}
\def\X@bibstep{[A(\the\c@X@bibstep)]}
\let\X@bibitem\bibitem
\def\bibitem{\global\advance\c@X@bibstep1\@ifnextchar[\X@bibitem{\expandafter\X@bibitem\X@bibstep}}
\begin{document}
\cite{c,e,b,a}
\begin{thebibliography}{9}
\bibitem{a} AAA
\bibitem{b} BBB
\bibitem{c} CCC
\bibitem{d} DDD
\bibitem{e} EEE
\end{thebibliography}
\end{document}