ProvidesFile{JohnDoePrivate.lco}[2012/04/11]
setkomavar{fromname}{John Doe}
setkomavar{signature}{John Doe}
setkomavar{fromaddress}{Iceroad 4\ Northpole}
...

documentclass[foldmarks=true,foldmarks=blmtP,%
JohnDoePrivate,version=last]{scrlttr2}
usepackage[utf8]{inputenc}

begin{document}
opening{Dear Peter,}

I just want to say "hi".

closing{Best wishes}
end{letter}
end{document}

~/texmf/[TDS]
/usr/local/texmf/[TDS]
/usr/local/texlive/texmf-local/[TDS]