Many image managers have the possibility to save the opened file to a different format like ISO, NRG, MDF/MDS, etc. As you will see in the image below, I have the option where I can select the output file format (ISO). The application used for the job is named UltraISO and it works flawlessly in trial mode. Simply open the .NRG file then use the main menu > File > Save As > Select ISO from the dropdown menu and input a name for the file. Depending on the size of the image, the whole process could take from a few seconds to a couple of minutes. In the image below I already have an opened file (.NRG format).