natbib same author same year: cite as “a”, “b” etc

Question

In the below file, I wanted the three references to be cited as Inger et al 2006a, Inger et al 2006b and Inger et al 2006c. This answer seems to achieve what I want, but requires fiddling with the .bib file. Is there a way to achieve this outside of the .bib file?

\documentclass{article}

\usepackage[comma, sort&compress]{natbib} 

\usepackage{filecontents}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@article{IngerEtAl2006a,
  title={Using daily ration models and stable isotope analysis to predict biomass depletion by herbivores},
  author={Inger, Richard and Ruxton, Graeme D and Newton, Jason and Colhoun, Kendrew and Mackie, Kerry and Robinson, James A and Bearhop, Stuart},
  journal={Journal of Applied Ecology},
  volume={43},
  number={5},
  pages={1022--1030},
  year={2006},
  publisher={Wiley Online Library},
  tag={}
}
@article{IngerEtAl2006b,
  title={Prey choice affects the trade-off balance between predation and starvation in an avian herbivore},
  author={Inger, Richard and Bearhop, Stuart and Robinson, James A and Ruxton, Graeme},
  journal={Animal Behaviour},
  volume={71},
  number={7},
 pages={1335--1341},
  year={2006},
  publisher={Elsevier}
}
@article{IngerEtAl2006c,
  title={Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis},
  author={Inger, Richard and Ruxton, Graeme D and Newton, Jason and Colhoun, Kendrew and Robinson, James A and Jackson, Andrew L and Bearhop, Stuart},
  journal={Journal of Animal Ecology},
  volume={75},
  number={5},
  pages={1190--1200},
  year={2006},
  publisher={Wiley Online Library}
}
\end{filecontents}
\begin{document}

    \cite{IngerEtAl2006a}\\\\
    \cite{IngerEtAl2006b}\\\\
    \cite{IngerEtAl2006c}

    \bibliographystyle{agsm}
    \bibliography{\jobname}
\end{document}