You don't have to make a backup but if you have any data you value it is unwise not to do so. Any time you mess with disk partitions or perform other disk activities you should do a backup. Something could go wrong and you could lose your data. In general you should always backup your data not just when creating a Boot Camp partition.
The reasoning is, very BAD things can happen. And if they do, you will be back here asking for a fix. The fix is to have a backup.
I urge you to protect your investment and your data (not to mention your sanity) and get an external drive.
Sorry I don't know a workaround, and there may be a good reason there isn't one.