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!
Good to know I'm not the only one. Do you still get this problem in the latest version of NPrinting? (April 2018 I believe, at time of writing.)
I'm still on NPrinting Nov 2017 and unable to upgrade to April (or Feb) 2018 for other, unrelated reasons, so I haven't tested this against the latest version. Consequently I haven't reported this formally to Qlik as a possible bug. (In any case, I worked around it by just using regular PowerPoint tables and accepting a slight loss of functionality.)
If you're still getting this problem with April 2018 release, I suggest it might be worth reporting to Qlik as a bug.