I'm creating a proposal for my client in Word, which has a header/footer/pagination on each page (8x10 page). They would like me to insert an 11x17 landscape page in the middle of the current document . Is this possible?

Yes. You can do that with brakes (page layout -> brakes)
To be more precise in brakes you must insert "next page" on the last line of a 8x10 page before that page you wish to be 11x17. Then you adjust desire size from 8x10 to 11x17. Notice that size of all pages below have changed. To fix that you must set another "next page" brake on the last line of that page you just changed. Then all you have to do is to set 8x10 on page under
11x17 page. And that is all.

For more understanding you can turn on " backward P " and see how this works. Remember that when you press enter on 11x17 the brake will go lower and lower and if he reaches next page it will change it to 11x17.

Sory for any translations errors im PL. ;p

edit: also remember that from now on your headers/footers will seperate on to 3 sections that each can be controlled manually. Default they are linked with each other so you won't see any difference between them, but if you smartly unlink them in header/footer options, you can even specify a different margin for page 11x17. But caution with that! backup your file before any of these changes!