[icesurf3r]

Thanks Dan 

I have found a solution, so here it is for anyone else that's interested.
Download Kodi Texture tool from https://kodi.wiki/view/TextureTool

To Update Exisiting Filmstrips

1. Run the program
   
2. Click Select input
   
3. Navigate to add-ons/resource.images.moviegenreicons.filmstrip-hd.colour/resources
   
4. Select Output Directory/ folder
   
5. Click Start
   
6. Add your additional movie genre icons to the output directory/ folder
   
7. Click Select Input Folder
   
8. Navigate to the direcorty/ folder
   
9. Select output file
   
10. Click start
    
11. copy/move textures.xbt from outout directory/ folder to add-ons/resource.images.moviegenreicons.filmstrip-hd.colour/resources (either overwrite the old file or rename it to keep a backup)
    

To add complete new set

1. Run the program
   
2. Click Select Input Folder
   
3. Navigate to the folder containing your custom movie genre icons
   
4. Select Output file
   
5. Clcik Start
   
6. copy/move textures.xbt from outout directory/ folder to add-ons/resource.images.moviegenreicons.filmstrip-hd.colour/resources (either overwrite the old file or rename it to keep a backup)
   

Should work for Kodi Matrix too.