Adding indentation after setting setlengthparindent{0pt}'

You should always be able to use hspace for this purpose: hspace{5mm}

EDIT:
In order for hspace to work you probably have to switch from vertical to horizontal mode using leavevmode. See Function and usage of leavevmode and What are vertical and horizontal modes?

So all in all this should work: leavevmodehspace{5mm}

First store the original indent by adding

newlength{saveparindent}
AtBeginDocument{setlength}{saveparindent}{parindent}}

to your preamble. Then also add

newcommand{indentpar}{parhspace*{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}{parhspace*{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}