XnView Software

I try to convert tga files with colors in tga files in grey levels but I need to keep the opacity of the picture.

I try to use like in XnView the saturation with the -hls argument of nconvert but when I do it :
"C:\program files\xnview\nconvert" -npcd 2 -size 256x256+0 -ctype grey -corder inter ...