Hi. You can only make attachments to an existing note using images (stored in your camera roll). Otherwise, it is not possible at this time.
The situation is exacerbated by Apple's decision that users don't need a file system, and with the eviscerated operating system I think you would need for the developers to be pretty determined to work around the obstacles to get this done. I can imagine ways in which they might accomplish it, but in the end, it will probably still be easier to wait until you get home to a "real" computer so that you can just drag and drop the attachments.
As someone who spends a lot of time working on his iPad, I find this state of affairs to be quite sad. I had hoped that Windows RT on the Surface would give me an alternative to the iPad, but there are so many issues there, I'd say it is not even close to being ready for prime time. Perhaps Windows Pro + Evernote will do the trick if/when it comes out. Or, maybe Apple will beef up things in iOS 7
The only alternative I know of at the moment is to remotely login to your desktop from your iPad. In fact, this is a pretty nice workaround, so you might want to give it some thought, even if it kind of defeats the purpose of using the iPad!