I can't add them in game either.
Add them in KFMod20.ini's [xVoting.xVotingHandler] section.
GameConfig=(GameClass="KFMod.KFGameType",Prefix="KF",Acronym="KF",GameName="Survival",Mutators="NoDoorDamage.NoDoorDamage,KFMod.Bloom,AdminPlus_v14.MutAdminPlus",Options=)
With this you can make sure the mutators are loaded every time a new map is voted.
For having certain mutators the first time you start the server, put them in cmd line.
You can't just extract mutators from cache, as some files are never downloaded from server. Just google "MutHolidayFunV20" if you want to use it.