Using El Capitan, can I create a bootable Windows 7 USB?
As @hirnwunde has pointed in the comments, the article from Indiana University works!


From the Finder, open the Applications folder, and then Utilities.
Open the Boot Camp Assistant, and then click Continue. The next screen should give you a list of options.
UITS recommends making the USB drive installer first. You don't need to install Windows or download the support software at this time, as it will be easier to do both later. To proceed:
 a. Verify that the USB drive you will be writing to is plugged in.
 b. Uncheck the Install Windows 7 or later version and Download the latest Windows support software from Apple options.
c. Check Create a Windows 7 or later install disk and click Continue.
Your USB drive should be listed in the "Destination disk" area. Use choose to browse to your .iso file; after selecting it, click Continue.
If prompted, confirm your action and/or provide an administrator's password. The process of writing the .iso file to the USB drive can take 20 minutes or longer.