There are a number of ways to achieve this, ranging from trivial to sophisticated.
Your question says that you would prefer to avoid an internet connection. Simple solutions may require it - if you want to abstract the difficult parts, you're going to have to let somebody do the dirty work, and that's probably going to be someone(thing) on the internet. Midrange solutions may require a network but not internet connection. A sophisticated solution could probably be whatever you want - but one thing I would say, is that trying to tap into the USB connector is either going to result in a hacktastic or very complicated solution to implement.
One method would be to integrate a third party framework that basically does what your looking for. Look at the Dropbox development kit, for example - allowing Windows (or any platform) users to drop files on their desktops into a shared dropbox, and this can then be read by an iOS application which includes the iOS drobox API.
Another method would be to setup a simple WebDAV server in your office. Host it on a windows box, or a cheap linux box. Give users desktop's access to the share via whatever protocol you want (eg, Windows File Sharing). Then you'd implement a WebDAV client in your app (eg, WTClient) to pull files.
Finally, you could build your own transmission system. A sophisticated example might involve Bonjour and TCP/IP, a simpler-but-custom solution may involve a simple JSON web service running off a local (or remote) server.
