At the moment iTunes can't add a folder to the file sharing area, only individual files. Looking at an iMovie project it is exported as a zip file with a .iMobileMovie extension and I don't really understand why the same hasn't been done with .band exports.
I have seen reports of success using iExplorer (formerly iPhoneExplorer) to add projects to the device but having completely failed to get it to work, or even show me the contents of the GarageBand app's folder on my iPhone I can't really suggest that as an option right now.
I have however submitted a bug report about the issue so hopefully it will be addressed at some point.
YEAAHH!! I did it! I just made a transfer of a .band folder from my PC to my iPhone through iExplorer!
This was my steps
1. Download iExplorer and install it. Connect your iPhone
2. Find the folder "com.apple.mobilegarageband" in iExplorers browser
3. Drag and drop your folder .band to the folder "Documents". (!! Your'e not done yet!)
4. In GarageBand on your iPhone press the plus-button as you would start a new song. Choose "Copy from iTunes". Choose your imported song.
5. Start rockin'
I've searched 5 hours to find this solution and man am I glad to finally have my songs back!!