If you have a bootable Intel Mac .dmg file containing a single uncompressed HFS+ disk image (e.g. Niresh Hackintosh USB .dmg files), just add the .dmg file to your E2B USB drive and keep the file extension as .DMG (or .dmgPTN). Example: Niresh OSX-Mavericks.dmg 5.26 GB (5,653,921,792 bytes).
If you have a bootable .dmg file containing a single compressed HFS+ partition image (e.g. MBR+1.hfs), then use 7Zip to extract the single partition image file (e.g. 1.hfs) and add it to your E2B USB drive (it can be renamed and given a .hfs or .hfsptn file extension). Example: Niresh Yosemite-Zone_10.10.1.dmg 4.96 GB (5,332,012,208 bytes) - extract 1.hfs.
Note: If you get a boot1:/boot error under VBox, try booting from a real system!
.dmg files that contain more than one partition are not currently supported.
You can create a USB Flash drive using Windows TransMac (right-click on USB drive - Restore with Disk Image) and then if it contains a HFS single partition, make a partition image .HFSptn file using RMPrepUSB - Disk->File - Start=P1, End=P1, FileStart=0). However, the .dmg image must be bootable and most Apple .dmg Installers/Upgrades are not bootable.
Note: When you run a .dmg or .hfs file, it creates a new partition entry #4. This partition entry is used by E2B and so is normally unused/empty.
E2B will complain about it when you reboot to the E2B drive and it will ask if you want to delete it (if you don't delete it then the E2B menu will not load!).
If you regularly use .dmg and/or .hfs files, you can add a 'partnew' command line to the \_ISO\MyE2B.cfg file to delete the #4 partition every time E2B boots. See the Sample_MyE2B.cfg file for more details.