If you are going to create a Keynote presentation on your iPad, you can use the "multi-tasking" feature of iOS to allow you to copy from your Pages copy and Paste into Keynote. Double-click on the home button to reveal a list across the bottom of your display showing other apps that are running. Tap on the other app to edit documents, repeat as needed.
I don't recommend bulk copy/paste as the content displayed in Keynote deserves to be well crafted for your intended audience and cramming the contents of a word processor onto a bunch of slides and calling it done would create a mess no one would enjoy seeing.
Use copy and paste judiciously to capture images of interest and perhaps headlines and charts. Leave the rest for others to read outside of your presentation. Good presentation display supports the speaker and doesn't repeat everything to be said. Keep your slide details to a minimum, use a legible typeface and keep your message large and simple. A few special transitions or builds can have a greater impact than too many.
With all that in mind, spending a few minutes with your original presentation and filtering out the extraneous for your Keynote and making a few notes on what to use would be time well spent.
Remember, there is a Help File under the Keynote Tools Menu (wrench) if you get stuck.