Solved the problem yourself:
In the file "header.php" before the
</ head>
added script:
<script type="text/javascript">
$(document).ready(function() {
$("a[href^=http]").each(
function(){
if(this.href.indexOf(location.hostname) == -1) {
$(this).attr('target', '_blank');
}
})
});
</script>