newcounter{rsss}

DeclareRobustCommand{appendixString}{
    setcounter{rsss}{value{chapter}}
    addtocounter{rsss}{- value{chapterCounter}}
    Anhang Alph{rsss} :
}
DeclareRobustCommand{appendixStringHeadline}{
    setcounter{rsss}{value{chapter}}
    addtocounter{rsss}{- value{chapterCounter}}
    addtocounter{rsss}{1}
    Appendix Alph{rsss} :
}

DeclareRobustCommand{appendix}[2]{
    addchap[appendixStringHeadline #1]{appendixStringHeadline     #1}label{#2}
    ihead{
    appendixString #1
    }
    stepcounter{chapter}
}

documentclass{scrreprt}
usepackage{scrpage2}pagestyle{scrheadings}

newcounter{romanSave}
newcounter{chapterCounter}
newcommand{switchToArabicPageCounter}{
    setcounter{romanSave}{value{page}}
    pagenumbering{arabic}
}
newcommand{switchBackToRomanPageCounter}{
    newpage
    pagenumbering{Roman}
    setcounter{page}{value{romanSave}}
    setcounter{chapterCounter}{value{chapter}}
}

newcounter{rsss}

DeclareRobustCommand{appendixString}{
    setcounter{rsss}{value{chapter}}
    addtocounter{rsss}{- value{chapterCounter}}
    Anhang Alph{rsss} :
}
DeclareRobustCommand{appendixStringHeadline}{
    setcounter{rsss}{value{chapter}}
    addtocounter{rsss}{- value{chapterCounter}}
    addtocounter{rsss}{1}
    Appendix Alph{rsss} :
}

DeclareRobustCommand{appendix}[2]{
    addchap[appendixStringHeadline #1]{appendixStringHeadline     #1}label{#2}
    ihead{
        appendixString #1
    }
    stepcounter{chapter}
}
begin{document}
tableofcontents
chapter{ Bla}
chapter{ Bla Bla}
appendix{test 1}{bla bla}
appendix{test 2}{bla}

end{document}