Character skins can be displayed and selected by players in the profile/models menu, and the scripts can also force skins to players during gameplay.
The asset files to import must be located in the folder {maniaplanet_user_dir}/Work/Skins/Models/{name_of_the_model}/{name_of_the_skin}
For the default shootmania model (aka 'the shootman'), {name_of_the_model} is ArenaPlayer
The asset files to provide are
See Sample skin : Work/Skins/Models/ArenaPlayer/SintelSample (in NadeoImporterSamples.zip)
To learn how to prepare the character mesh for import, see the dedicated page : mesh import