First store the original indent by adding
\newlength{\saveparindent}
\AtBeginDocument{\setlength}{\saveparindent}{\parindent}}
to your preamble. Then also add
\newcommand{\indentpar}{\par\hspace*{\saveparindent}\ignorespaces}
as your new indentation mechanism.
Assuming you have \setlength{\parindent}{0pt}
somewhere else in your document you should now be able to use it in the following way:

\documentclass{article}
\newlength{\saveparindent}
\AtBeginDocument{\setlength{\saveparindent}{\parindent}}
\newcommand{\indentpar}{\par\hspace*{\saveparindent}\ignorespaces}
\begin{document}
This is an indented paragraph.
\setlength{\parindent}{0pt}% Remove paragraph indentation
This is a paragraph that is not indented.
\indentpar
Here is a new indented paragraph.
\end{document}
Of course, LaTeX also provides \noindent
, if you're only interested in some non-indented paragraphs:
\documentclass{article}
\begin{document}
This is an indented paragraph.
\noindent% Remove paragraph indentation just for this paragraph
This is a paragraph that is not indented.
Here is a new indented paragraph.
\end{document}
\hspace*
so the space is not dropped but I would not redefine the\indent
primitive, define your own command.