with your hints I have been able to make significant progress. Now I can transfer text from Excel into PowerPoint. The problem I have to solve is, that I only can create a new slide at the end of my exisiting template and write some new text into that. Code for that section I am using:

Java Code:
    public void modSlide1() throws IOException {
    	XlsReader xls = new XlsReader(XLS_SOURCE);
    	Slide slide = ppt_.createSlide();
    	addText(slide, xls, 0, 0);
In thought I find the information in poi-3.7, but cannot find anything. Anyone how can help is much appreciated.