Maybe I'm not understanding your question?
I believe on Macs that shipped with SuperDrives, the legacy BIOS support only allows booting from the SuperDrive. It is possible to work around this, but it's for more advanced users. It is technically possible to install Windows 7 FROM USB media on this iMac, but it is not officially supported and will not work with Apple's implementation of Boot Camp and their EFI architecture.
If Boot Camp does not give you an option for creating a Windows 7 USB install media, it is NOT officially possible to boot from a Windows installation USB drive with your iMac.
If you're wanting to install Windows 7 on USB media, it is not possible without extensive modifications to the Windows 7 installation and cannot be installed under normal means. Even if you managed to make it work, you would have to either use rEFInd and probably manually create the hybrid MBR (terrible hack, why Apple?)
Windows 8 introduced the ability to be installed to and boot from USB media, but ONLY if the drive is recognized as a fixed disk and NOT a removable disk in Windows. Windows can only recognize the first partition on removable media.
Can you further elaborate what you're seeking to do? If what I explained is hard to follow, please let me know and I can tone it down a bit.