The instructions assume the players have understood the basic installation and operation of SourceMod platform. If you never installed SourceMod in your dedicated server before, search the instructions about installing SourceMod.

[L4D & L4D2] Mission and Weapons - Info Editor (https://forums.alliedmods.net/showthread.php?t=310586) is a powerful SourceMod plugin, which can easily modify the mission file without extracting or altering the VPK addon. It could help you join custom melee weapons in most community campaigns.

After you installed the Mission and Weapons - Info Editor, edit l4d_info_editor_mission.cfg

Left 4 Dead 2 Dedicated Server\left4dead2\addons\sourcemod\data\l4d_info_editor_mission.cfg

and find the "meleeweapons". You could create a short-cut of l4d_info_editor_mission.cfg on desktop for handy editing later.

To modify "meleeweapons" and join all custom melee weapons. You could also join the riot shield (riotshield) if it's available in your server and no need to join chainsaw in following list. Please note that the maximum quantities of melee weapons are 16 so you have to omit some default melee weapons if the custom weapons in the campaign + weapons of Custom Weapon Base + default melee weapons exceed the limit.
Some of community campaigns have their own custom melee weapons that are random spawning so you had better check them with GCFScape (http://nemesis.thewavelength.net/?p=26) before playing any community campaigns.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1881573497 The mission file sometimes defines the "meleeweapons" which are from the author's preference. Supposing you don't find any "meleeweapons" in the mission file, that means the campaign adopts the default settings of melee weapons so you could join any melee weapons you like without worry.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1881574926 The latest Mission and Weapons - Info Editor now supports that custom weapons in the campaign are automatically added on top of your "meleeweapons" string. For instance, if you set following string in the l4d_info_editor_mission.cfg and then play the Blood Tracks campaign:
The plugin will join the exclusive weapons in the Blood Tracks campaign actively and override the mission file like following example:
Please note that not every player joining your server has Custom Weapon Base, so you must leave some basic melee weapons for them. You may delete the melee weapons you don't like and let other melee weapons be more possible to appear.
Certain of community campaigns, like Dark Wood Extended and Resident Evil series by Roku, have their custom melee weapons for rewards of easter egg. These melee weapons are constant positions and you cannot find them in randomly spawning positions. They may be replaced by the melee weapons of Custom Weapon Base even though joining their names in the list of "meleeweapons", so you ought to turn it off while playing these maps. To do this, putting 2 of slash characters ( // ) before it will stop the plugin changing them.
You could put many "meleeweapons" in l4d_info_editor_mission.cfg but remember: only use one of them at a time.

//"meleeweapons" "baseball_bat;crowbar;fireaxe;frying_pan;katana;tonfa;pitchfork;shovel;barnacle;bow;custom_ammo_pack;custom_shotgun;flamethrower;m72law;sentry;syringe_gun" //Alternative 1 (Default melee weapons (6+2) + custom weapon base (8) <= 16)
//"meleeweapons" "cricket_bat;electric_guitar;golfclub;machete;knife;pitchfork;shovel;barnacle;bow;custom_ammo_pack;custom_shotgun;flamethrower;m72law;sentry;syringe_gun" //Alternative 2 (Default melee weapons (5+2) + custom weapon base (8) <= 16)

Alternative 1&2 can be used in any campaigns without custom melee weapon. Remember to delete some default weapons in community maps if the total of melee weapons > 16.