You cannot
The eaw.exe is programmed to read and load skin files based on the names of the 3dz files . When we introduced multiskins the code-group's exe had to be modified with additional code to read the names of the multiskin files.
In one of the mustang multiskin folders there is a "PP51DF.3dz" file which 1.2 and the GoG versions are programmed to read.
There is also a "PP51D01F.3dz", and the "PP51D02F.3dz" ..........."PP51D15F.3dz" files which are the multiskin variations of the "PP51DF.3dz" file.
The 1.2 and the GoG versions are not programmed to read these additional 15 names.
The GoG version does not have this additional code. It is basically a nostalgia trip for people who had eaw V1.2 but could not run it on their newer PCs.