\documentclass{article}

\usepackage{fontspec}
\usepackage{unicode-math}

\ExplSyntaxOn
\tl_gput_right:Nn \g__um_mathtable_tl
  { \__um_sym:nnn {"E1201} {\mychardisp} {\mathop} }
\ExplSyntaxOff

\ExplSyntaxOn
\tl_gput_right:Nn \g__um_mathtable_tl
  { \__um_sym:nnn {"E1200} {\mychar} {\mathop} }
\ExplSyntaxOff

\setmainfont{TeX Gyre Pagella}
\setmathfont{TeX Gyre Pagella Math}

\setmathfont[
  Path=./,
  range={\mychar,\mychardisp}
]{PagellaCompanion-Regular.otf}


\begin{document}
$\sum \mychar$



$\sum_{i=1}^2 i^{-1}= \mychar_{i=1}^2 i^{-1}$

$\symup{K=K^2=K_2K=K}$

\[\sum\mychardisp\]
\[\sum_{i=1}^{\infty}\frac{1}{i}\  =  \mychardisp_{i=1}^{\infty}\frac{1}{i} \]
\end{document}