Line breaks are a very delicate, fragile and complex matter.
If you have problems with your line breaking I would suggest trying the following solutions in the order listed here.
The package microtype
can often help with line breaks and other micro-typographic features. It cannot help with all problems, the issue in the MWE it is well able to resolve.
\documentclass[parskip=half,12pt]{scrreprt}
\usepackage[a4paper, margin=2.54cm, marginparwidth=2.0cm, footskip=1.0cm]{geometry}
\usepackage{mathptmx}
\usepackage{microtype}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@book{xyz,
title={Iasd},
author={Kersten, X. and Bjhsdfojgf, A.D. and HBjdsf, J. and Sjkvhds, K.W.},
year={2013},
}
\end{filecontents*}
\usepackage[backend=biber,
style=alphabetic,
]{biblatex}
\addbibresource{\jobname.bib}
\begin{document}
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed dm nonumlllllllllllll \cite[270]{xyz}. invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebu. clita kasd gubergren, no sea takimata
Pino dfgdfgfdgfgfgfgf sdsdsds den dfgdfgfgfgfgfg eines Datensatzes irreversibel \cite[270]{xyz}. invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebu. clita kasd gubergren, no sea takimata
\end{document}

Then you can reword your sentences or rearrange your text such that the line breaks look fine afterwards.
If microtype
cannot help you and you don't want to rearrange your text, you might have to resort to more ad-hoc solutions.
You can allow for additional stretch space via \emergencystretch
.
You will probably have to experiment with the value, in your MWE
\setlength{\emergencystretch}{2pt}
worked just fine.
See also What is the meaning of \fussy, \sloppy, \emergencystretch, \tolerance, \hbadness?, Temporarily increase the limit on space size? and How to use \sloppy for just some references?
\documentclass[parskip=half,12pt]{scrreprt}
\usepackage[a4paper, margin=2.54cm, marginparwidth=2.0cm, footskip=1.0cm]{geometry}
\usepackage{mathptmx}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@book{xyz,
title={Iasd},
author={Kersten, X. and Bjhsdfojgf, A.D. and HBjdsf, J. and Sjkvhds, K.W.},
year={2013},
}
\end{filecontents*}
\usepackage[backend=biber,
style=alphabetic,
]{biblatex}
\addbibresource{\jobname.bib}
\setlength{\emergencystretch}{2pt}
\begin{document}
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed dm nonumlllllllllllll \cite[270]{xyz}. invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebu. clita kasd gubergren, no sea takimata
Pino dfgdfgfdgfgfgfgf sdsdsds den dfgdfgfgfgfgfg eines Datensatzes irreversibel \cite[270]{xyz}. invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebu. clita kasd gubergren, no sea takimata
\end{document}

The higher \emergencystretch
can be changed locally with the suggestions in An environment for setting \emergencystretch locally
\newenvironment{emergency}[1]{%
\par
\setlength{\emergencystretch}{#1}%
}{%
\par
}
You then wrap the environment emergency
with a suitable length in the argument around the paragraph having breaking troubles.
\documentclass[parskip=half,12pt]{scrreprt}
\usepackage[a4paper, margin=2.54cm, marginparwidth=2.0cm, footskip=1.0cm]{geometry}
\usepackage{mathptmx}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@book{xyz,
title={Iasd},
author={Kersten, X. and Bjhsdfojgf, A.D. and HBjdsf, J. and Sjkvhds, K.W.},
year={2013},
}
\end{filecontents*}
\usepackage[backend=biber,
style=alphabetic,
]{biblatex}
\addbibresource{\jobname.bib}
\newenvironment{emergency}[1]{%
\par
\setlength{\emergencystretch}{#1}%
}{%
\par
}
\begin{document}
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed dm nonumlllllllllllll \cite[270]{xyz}. invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebu. clita kasd gubergren, no sea takimata
\begin{emergency}{1pt}
Pino dfgdfgfdgfgfgfgf sdsdsds den dfgdfgfgfgfgfg eines Datensatzes irreversibel \cite[270]{xyz}. invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebu. clita kasd gubergren, no sea takimata
\end{emergency}
Pino dfgdfgfdgfgfgfgf sdsdsds den dfgdfgfgfgfgfg eines Datensatzes irreversibel \cite[270]{xyz}. invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebu. clita kasd gubergren, no sea takimata
Lorem \cite[270]{xyz} ipsum
\end{document}

microtype
package which can help in many (but unfortunately not all) cases - in your MWE it helps. Then you could try\emergencystretch
(see here and here) - maybe even only locally.... – moewe Sep 2 '15 at 9:35microtype
package removed all ugly linebreaks but your other suggestions work as well. Can't decide for one solution yet but the problem is fixed. If you like, sum it up in an answer. It might be accepted ;) – hurb Sep 2 '15 at 10:12\penalty
in\postnotedelim
. The required penalty was so negative, it forced line breaks virtually everywhere, which rendered this approach quite useless, obviously. – moewe Sep 2 '15 at 10:29