I do not know how in Windows, but this time there is something non-standard with NoMouth.dat - it can't be easily just renamed to .iso and mounted (in Mac OSX at least).
But as it works in DOSBOX, you can mount and extract files directly in DOSBOX itself (mount your GOG folder with IHNMAIMS, mount properly NoMouth.dat, make some directory (e.g. ISO) and copy files there:
Z:\>mount c /your_path_to/IHNMAIMS (e.g.: mount c .. -when you started Dosbox.exe from installed DOSBOX folder inside your game folder)
Z:\>imgmount d -t iso -fs iso C:\NOMOUTH.DAT
Z:\>copy D:\*.* C:\ISO\
now you should have ISO directory inside your game directory with all files extracted, and now you can choose this directory for installing game in ScummVM
What does the -fs parameter do in this context? I cannot find this information anywhere, not even on the dosbox wiki.
it specifies resulting file system format. Just for extracting, I think you can choose fat too...
IMGMOUNT DRIVE [imagefile] -t [image_type] -fs [image_format]
-size [sectorsbytesize, sectorsperhead, heads, cylinders -u DRIVE]
The following are valid file system formats:
Specifies the ISO 9660 optical disc format.
Specifies that the image uses the FAT file system. DOSBox will attempt to mount this image as a drive in DOSBox and make the files available from inside DOSBox.
DOSBox will make no attempt to read the file system on the disk. This is useful if you need to format it or if you want to boot the disk using the BOOT command. When using the "none" filesystem, you must specify the drive number (2 or 3, where 2=master, 3=slave) rather than a drive letter. For example, to mount a 70MB image as the slave drive device, you would type: "imgmount 3 d:\test.img -size 512,63,16,142 -fs none" (without the quotes). Compare this with a mount to read the drive in DOSBox, which would read as: "imgmount e: d:\test.img -size 512,63,16,142".