For this numerki.tex file:
$x^2=\ln y$
zzzero
jEden
dWa
trZy
cztery
...
dwadzIescia
etc......
it works:
\documentclass{article}
\usepackage{readarray}
\usepackage{lipsum}
\renewcommand\thepage{\csname arg\roman{page}\endcsname}
\begin{document}
\readdef{numerki.tex}{\mypagenum}
\getargsC{\mypagenum}
\section{Test chapter one}
\lipsum[1-8]
\section{Test chapter two}
\lipsum[1-8]
\end{document}
Confirmed it works for math mode in pages, since user mentioned that.

This alternate version below, while a bit longer in code, will allow the use of regular spaces in your input lines without requiring you to change them to hardspaces.
\documentclass{article}
\usepackage{readarray}
\usepackage{lipsum}
\makeatletter
\renewcommand\readdef[2]{%
\def\first@row{T}%
\def\first@plane{T}%
\catcode\endlinechar=10\relax%
\def#2{}%
\newread\file%
\openin\file=#1%
\loop\unless\ifeof\file%
\read\file to\fileline % Reads a line of the file into \fileline%
\protected@edef#2{#2\fileline}%
\if T\first@row\getargsC{#2}\setcounter{@col}{\numexpr(\narg-1)}%
\edef\ncols{\arabic{@col}}\def\first@row{F}\setcounter{@row}{1}%
\else%
\if T\first@plane\getargsC{\fileline}\ifthenelse{\equal{\narg}{1}}{%
\edef\nrows{\arabic{@row}}\def\first@plane{F}}{%
\addtocounter{@row}{1}}%
\fi
\fi%
\expandafter\protected@edef\csname record\roman{@row}\endcsname{%
\fileline}%
\repeat%
\closein\file%
\catcode\endlinechar=5\relax%
}
\makeatother
\renewcommand\thepage{\csname record\roman{page}\endcsname}
\begin{document}
\readdef{numerki.tex}{\mypagenum}
\section{Test chapter one}
\lipsum[1-8]
\section{Test chapter two}
\lipsum[1-8]
\end{document}