setkomavar{firsthead}{
    scriptsize
    test\
    begin{tabular}[b]{p{57.5mm}}
        ttest
    end{tabular}
}

documentclass[]{scrlttr2}

makeatletter
@setplength{firstheadhpos}{142.5mm}
makeatother
setkomavar{firsthead}{
    scriptsize
        test\
        begin{tabular}[b]{p{57.5mm}}
        ttest
        end{tabular}
}

begin{document}

begin{letter}{Test}
opening{Hi}

end{letter}
end{document}