I think your question is really just a Microsoft PowerPoint question, not an NPrinting one. "How do I embed a pre-existing Excel spreadsheet file in a PowerPoint slide?"
I don't believe it's possible to do that directly (in any PowerPoint file, nothing to do with NPrinting). However surely you can just embed a new Excel Spreadsheet using the button you've shown in your screenshot, and paste in the content from your pre-existing spreadsheet that exists separately?
One word of warning: at the time of writing I am attempting to embed Excel spreadsheets in a Qlik NPrinting PowerPoint report (just as you want to do) and am finding that PowerPoint (the instance 'within' Nprinting) crashes consistently whilst doing this. (Nprinting Nov 2017 version.) A standalone instance of PowerPoint (outside NPrinting) doesn't crash following the same steps. I found your question whilst searching to see if I'm the only person experiencing this: whether it's me or a software bug. I'd be really grateful, if you try this out, if you can reply back if you also get crashes!