I want to add a movie from my iTunes to an iPad so it appears in the iPad's Camera Role, not in the 'Videos' app. I can't sync the iPad because it'll wipe all the info that is on it. How do I do this?

I am pretty sure Camera Roll (not role) is only used for items taken with the iPad. A different area is used for storing items synced to an iPad and you can't change that. As for syncing, you have left out some of the story because you only get that message when you connect the iPad to a different iTunes library to that used to sync the last time you synced the iPad. You need to connect the camera to the same library. Your i-device was not designed for unique storage of your media. It is not a backup device and media transfer was planned with you maintaining a master copy of your media on a computer which is itself independently backed up against loss. To use a device with a different setup you transfer the old library from a computer or a backup directly to the new setup, not the device to the library. Media syncing is one way, computer to device, updating the device content to the content on the computer, not updating or restoring content on a computer. The exception is iTunes Store purchases which can be transferred to a computer.

iTunes Store: Transferring purchases from your iOS device or iPod to a computer - http://support.apple.com/kb/HT1848 - only purchases from iTunes Store

For transferring other items from an i-device to a computer you will have to use third party commercial software. See this document by turingtest2: Recovering your iTunes library from your iPod or iOS device - https://discussions.apple.com/docs/DOC-3991