Actually if you are using a Mac, it doesn't matter the file will be formatted in .iPSW but problem comes when your OS is Windows. But in regard of sharing information I still share the software created to convert DMG based format file for Mac. Who knows you may want to know about them.
For example, if Apple releases a beta version of their mobile operating system which is only for registered developers. And the iOS beta versions are usually released in DMG formats therefore Windows OS users can't use the versions.
So if you wish to convert the DMG file format into IPSW to which you can use the file to upgrade your iPhone or iPad in iTunes. This post containing a few methods you can utilize to convert or extract the DMG format to iPSW format using a third party apps like 7-Zip, DMG extractor, TransMac or PowerISO in Windows and Mac.
Easily convert DMG file to IPSW format using great 4 tools
1. 7-Zip (Windows)
7 Zip is a tool to convert / extract the downloaded DMG file for free.
Download 7-Zip (Windows)
To use 7-zip in Windows: Choose File > Open then point to DMG (format) file in PC then select extract button.
2. DMG Extractor
DMG extractor can be used in PC, but this application has two options which is the free and premium versions. You can use the free version if you just want to extract the DMG to IPSW format.
Download DMG Extractor (Windows)
To use DMG extractor just choose Open button and then Extract.
3. TransMac (Windows and Mac)
TransMac is a tool to extract file and can be used in Windows or Mac. Beside that, with TransMac can also compress the file, however to use it you must pay, but there is 15 days free trial you can try.
Download TransMac (Windows and Mac)
4. Power ISO
Power ISO is an extract file tool that can be used to burn, split, and encrypt file easily. PowerIOS can be downloaded for free only for Mac, but not-free for Windows OS.
Download PowerISO (Windows and Mac)
The step to use PowerISO in Windows and Mac is almost the same like using the above tools.
That's all. The above 4 tools are file converting tools you can use to convert .dmg file to .ipsw file easily for Windows or Mac. If you have other software that you can consider very good, why not sharing it to us.