Is there a way to export non-dithered tga from Designer?

If I need some patches of flat gray values  in one of rgba   working as time values  .   So far I have to export as 16 bit  tiff  and convert it to 8 bit  in Photoshop   with dithering off.      Can I do same in Designer somehow ?   When I try  to export tga  from Designer directly  my flat gray areas always get some dithering  noise.