Can I use iTunes to sync a folder of photos onto my iPhone without losing my iPhone Camera Roll?
Yes, you can safely sync a specific folder from your Mac to your iPhone via iTunes without overwriting your iPhone's Camera Roll.
You don't specify your version of iOS, but typically what will happen is that the sync will create an additional album in the Photos app on your iPhone. So, using your example, if you use iTunes on your Mac to sync selected albums and then select the "Pictures From Brazil" folder, this will add an album on your iPhone called "Pictures From Brazil".
This additional album doesn't impact your Camera Roll at all.