You can edit Third_Age_3\data\export_descr_buildings.txt:
Put these in the barracks buildings, under other "recruit_pool" lines. The following lines are for DaC submod which have more factions, so you need to remove/rename some of them. If it contains any unit which doesn't exist, M2TW would fail to run and you can check the error from logs.
Code:
recruit_pool "Gondor Bodyguard" 0 0.1 1 0 requires factions { sicily, united, }
recruit_pool "Amroth Bodyguard" 0 0.1 1 0 requires factions { denmark, united, }
recruit_pool "Rohan Bodyguard" 0 0.1 1 0 requires factions { milan, }
recruit_pool "Breeland Bodyguard" 0 0.1 1 0 requires factions { normans, }
recruit_pool "Dunedain Bodyguard" 0 0.1 1 0 requires factions { turks, united, }
recruit_pool "Dunedain Aragorn Bodyguard" 0 0.1 1 0 requires factions { turks, united, }
recruit_pool "Dale Bodyguard" 0 0.1 1 0 requires factions { scotland, }
recruit_pool "Anduin Bodyguard" 0 0.1 1 0 requires factions { timurids, }
recruit_pool "Khazad Balin Bodyguard" 0 0.1 1 0 requires factions { norway, }
recruit_pool "Erebor Bodyguard" 0 0.1 1 0 requires factions { moors, norway, }
recruit_pool "Ered Luin Bodyguard" 0 0.1 1 0 requires factions { hungary, }
recruit_pool "Lindon Bodyguard" 0 0.1 1 0 requires factions { saxons, }
recruit_pool "Imladris Bodyguard" 0 0.1 1 0 requires factions { egypt, }
recruit_pool "Woodland Bodyguard" 0 0.1 1 0 requires factions { mongols, }
recruit_pool "Lorien Bodyguard" 0 0.1 1 0 requires factions { ireland, }
recruit_pool "Dorwinion Bodyguard" 0 0.1 1 0 requires factions { byzantium, }
recruit_pool "Enedwaith Bodyguard" 0 0.1 1 0 requires factions { teutonic_order, }
recruit_pool "Dunland Bodyguard" 0 0.1 1 0 requires factions { aztecs, }
recruit_pool "Mordor Bodyguard" 0 0.1 1 0 requires factions { england, }
recruit_pool "Goldur Nazgul Bodyguard" 0 0.1 1 0 requires factions { poland, }
recruit_pool "Isengard Bodyguard" 0 0.1 1 0 requires factions { france, }
recruit_pool "Misty Bodyguards" 0 0.1 1 0 requires factions { hre, }
recruit_pool "Angmar Bodyguards" 0 0.1 1 0 requires factions { portugal, }
recruit_pool "Umbar Bodyguard" 0 0.1 1 0 requires factions { russia, }
recruit_pool "Harad Bodyguard" 0 0.1 1 0 requires factions { spain, }
recruit_pool "Rhun Bodyguard" 0 0.1 1 0 requires factions { venice, }
recruit_pool "Khand Bodyguard" 0 0.1 1 0 requires factions { khand, }
The list of general bodyguard units can be found in Third_Age_3\data\export_descr_units.txt. General units are those with "general_unit" in unit attributes.