I would suggest to load package scrlayer-scrpage which is part of the KOMA-Script bundle.
If the footer on the second page should be at the same height as the first:
\documentclass{scrlttr2}
\usepackage[T1]{fontenc}
\usepackage[english]{babel}
%% Same left margin for everything
\setplength{toaddrhpos}{\dimexpr\oddsidemargin+1in}
%% Add a footer
\setkomavar{firstfoot}{\parbox[b]{\useplength{firstfootwidth}}{\centering foo | bar}}
\usepackage[manualmark]{scrlayer-scrpage}
\KOMAoptions{footwidth=\useplength{firstfootwidth}}
\ForEachLayerOfPageStyle{scrheadings}{%
\Ifstrstart{#1}{scrheadings.foot}{%
\ModifyLayer[
voffset=\useplength{firstfootvpos}
]{#1}
}{}%
}
\clearpairofpagestyles
\cfoot{\usekomavar{firstfoot}}
\addtokomafont{pagefoot}{\normalfont}
%% Move date to the left
\KOMAoptions{refline=dateleft}
\usepackage{lipsum}
\begin{document}
\begin{letter}{addressee}
\opening{Dear XXX}
\lipsum[1-3]
\closing{Yours}
\end{letter}
\end{document}

If the footer on the first page should be at the same height as on other pages (see also https://tex.stackexchange.com/a/578081):
\documentclass{scrlttr2}
\usepackage[T1]{fontenc}
\usepackage[english]{babel}
%% Same left margin for everything
\setplength{toaddrhpos}{\dimexpr\oddsidemargin+1in}
%% Add a footer
\usepackage[manualmark]{scrlayer-scrpage}
\clearpairofpagestyles
\cfoot*{foo | bar}
\addtokomafont{pagefoot}{\normalfont}
\newcommand{\originalopening}{}
\let\originalopening\opening
\renewcommand{\opening}[1]{\originalopening{#1}\thispagestyle{plain}}
\KOMAoptions{firstfoot=false}% disable first footer
%% Move date to the left
\KOMAoptions{refline=dateleft}
\usepackage{lipsum}
\begin{document}
\begin{letter}{addressee}
\opening{Dear XXX}
\lipsum[1-3]
\closing{Yours}
\end{letter}
\end{document}
