to the xcom1 txt file i saw here thats a bit screwed in my opinion
there is the color "48, 0, 0," three times - the color data generated from the palette data (for ufo battlscape) from warboy there are no color indices with the same values as mentioned here https://openxcom.org/forum/index.php/topic,1321.msg32646.html#msg32646
there are ambiguous colors in vanilla xcom palettes. it's how they are. for sdl screwed algorithms tho, warboy had to disambiguate them. but with that they stop being vanilla.
i see only 2 different conversion modes - without dithering, and with patterned (ordered) dithering. but i wouldn't suggest using dithering at all, better to edit them by hands, if anything.
bladum, my editor is capable of handling PCKs only (32x40 pixels), either for MCDs, or for units. You can save the set into png sheets with one palette, change current palette, and import png sheet.