Selection of PowerPointSlide # NOT create new slide

Hello,
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:

Code:

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    public void modSlide1() throws IOException {
            XlsReader xls = new XlsReader(XLS_SOURCE);
            Slide slide = ppt_.createSlide();
            addText(slide, xls, 0, 0);

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In thought I find the information in poi-3.7, but cannot find anything. Anyone how can help is much appreciated.
Thanks
Greenhorn.