as far as face texture maps go don't know.
you might be able to add it as a hair object. haven't actualy tride adding a new hair texture map yet. you may have to mess with scripting or something to get the textures to show up as the color is done on a slider instead of a button list
to added it as a hair import your helm like usual into a brf then open up skins.txt and find this block
not you will have to do this for male female and each additional race in your mod
20 -increase this by one for every helmet you add
man_hair_s man_hair_m man_hair_n man_hair_o man_hair_y10 man_hair_y12 man_hair_p man_hair_r man_hair_q man_hair_v man_hair_t man_hair_y6 man_hair_y3 man_hair_y7 man_hair_y9 man_hair_y11 man_hair_u man_hair_y man_hair_y2 man_hair_y4 -add your helmets here
next find this block
5 -increase this by one for every texture hair_blonde hair_red hair_brunette hair_black hair_white -add textures here
[example]
8
woman_hair_p woman_hair_n woman_hair_o woman_hair_q woman_hair_r woman_hair_t woman_hair_s Hairtmp
next open module.ini and change the third from last setting from this
limit_hair_colors = 1
to this
limit_hair_colors = 0
you could also probably do the same by adding them as beards since they are handled mostly the same (defined in same file and stuff), just md3's instead of obj's and they are mirrored in the game so chop your helms in half look in beards.brf for model examples.