Is there a way to add a "Next" button on all slides of an existing project (about 100 slides), preferably not having to alter the Master slide?
Master slide is the best solution, don't know why you exclude that? You don't tell if the Next button needs a pausing point? The advantage of such a pausing button on the master slide(s) is that the pausing point will always be at the end of each slide, whatever its duration.
Alternative: put the Next button (shape, bitmap image or SVG) on the first slide where you need it. Time that button for the rest of the project, and check the option 'Always on top'. Not the same advantage for the pausing point of the button, so you'll have to be careful.
SO I'm working with someone else's existing (and very old) project of about 100 slides. I should have included this detail on my post but, on each slide, the next button could show immediately/be delayed/point to something other than next slide/not required. So the action may be different for some slides but most will simply be the "next slide".
Basically, I'm just investigating if I can bulk load a Next button on all slides to save me some headache/time and then change the action on the ones I need to. If I add a Next button to the Master slide, can I change the action properties on select slides?
Indeed missing information is increasing time to answer. I have also no idea how many master slides are used in that project neither.
Go for a hybrid solution:
Timing for the rest of the project would make it too complicated in this situation. I mentioned it because you didn't want it on the master slide.
YES! Just place ONE button on the first slide and place it where you want it to be, and add the correct Action (go to next slide). Then you can just copy the button, select the first slide, hold down the shift key and selected the last slide, and paste and BOOM -- all your slides will have the same button in the same place.
However, you will still have to check each slide and see where the button will pause if the slides have different timelines and durations. For this reason a more practical method would be to place a button on your MASTER slide. You only have to place it ONCE, and the pause will occur at the END of each timeline no matter its duration. So this is the quick, simple and smart way to work!
Hope this helps!