On the mission text file, there's something like "survivor_set". Change this one to change which set of survivors will be available. You can ask further on "Modding/ Mapping" section, as questions about making mods & maps usually go there.

is it this one? "namvet_player_name_x" "-105"
"namvet_player_name_y" "-60"

"manager_player_name_x" "95"
"manager_player_name_y" "-30"

"teenangst_player_name_x" "22"
"teenangst_player_name_y" "8"

"biker_player_name_x" "-22"
"biker_player_name_y" "-95"

"character_order" "biker;namvet;teenangst;manager"