I am not sure you need paper sketch-style slides to make your presentation look less formal. I would loosen up the existing design, cutting words, changing language, etc. One way to do this is to find another presentation somewhere with an informal style you like, now copy the style of the template and re-create your existing presentation so that it exactly fits this model. I am pretty confident that in the end, you will discard your "formal" presentation all together and run with your new deck in all meetings (both formal and informal).

If you insist on making "sketchy" presentations, here are two approaches:

1) Apple Keynote has rougher border styles. combine this with a custom hand-writing-style font
2) The manual way: recreate your presentation but leave selected bits out (boxes, maybe certain charts, arrows), print it, take a big marker, draw in the elements, scan the thing to PDF. It will look really good, but is hard to edit.
3) A variant on this: take a print out of the half-finished deck to your meeting, and complete the charts on the spot, "live". This could work in a meeting setting.

Hope this helps.