As a start-of-a-paragraph "undent", this is easily achieved using a \makebox[0pt][r]{..} alignment:

\documentclass{article}
\begin{document}
\setlength{\parindent}{0pt}
My text. \par
My text. \par
\makebox[0pt][r]{My }special Text. \par
My text. \par
\noindent\llap{My }special text.
\end{document}
This sets the text in a box of width 0pt (avoiding overfull \hboxes) and right-aligned, pushing it into the left margin. You could also use \llap, which does the same, but you need to "start" the paragraph first before this takes effect.
It would also be possible to allow this "undent" mid-paragraph.