yes and no
yes you can add a custom face, but no, it can't just go along with the mission file. it requires you to create your own addon. before you sigh and mutter into your hands, i should inform you that it's relatively straightforward.
create your face files per the usual template, i.e. 128 x 128 pixels. you can leave them as jpg files, no need to convert to paa. stick them all in a folder and call the folder something.
then open up notepad and copy/paste the code in the attached file, changing the text to your filenames. save that file as config.cpp, stick it into the folder with all the faces, and then run
amalfi's pbo maker, select the folder and run it.
voila. one pbo addon, which when added to the addons directory allows you to use the names you defined in the config file in the usual way, i.e.
unit_name setface "name_of_face".any questions?