.APP file format is an application format for Mac OS X operating system. It is an archive file which contains the executable binary file along with the other resources that are required for proper execution of the application.
Files with the DMG extension are bootable disc images, supported in Mac OS X systems. This files are often encountered with installers of software downloaded from the internet. Such files are opened with the Apple Disk Utility application, which is used for mounting images in Apple devices.
After converting the file to the new format, you will be able to use other programs to support it. Keep in mind, however, that the APP file after converting to DMG may differ slightly from the original. The most important information should be kept, but if you want the file after conversion from APP to DMG to be the same, you have to do this carefully and choose the proper application from the list below.
There is no guarantee, of course, that the conversion will come up to all your expectations, but it will certainly help you. If, after all, the effect of converting APP to DMG does not meet your expectations, you can simply try to find on the Internet a different version of your file in APP format, properly converted to a DMG file by someone else. If this fails, use the information provided in the further part of the website.
Unfortunately, if after performing the two previously described steps (trying to find your APP file converted by someone else, and the attempt to convert it to DMG on your own) you still have a problem with your file, then there are only a few solutions left. You can once again try to find and install an application that can open the APP file in its original format (without conversion to DMG file). Such a solution will be difficult to implement, but it certainly will give the best result.