The files needed to edit, and what to add, correspondingly:
text\db\dilemmas.loc
Code:
dilemmas_localised_first_choice_title_d_bos_treaty_units_all Accept True
dilemmas_localised_second_choice_title_d_bos_treaty_units_all Refuse True
dilemmas_localised_title_d_bos_treaty_units_all Western Military Missions True
dilemmas_localised_first_choice_label_d_bos_treaty_units_all Such a military understanding will be a great asset to our cause. We accept. True
dilemmas_localised_description_d_bos_treaty_units_all Having had long relation with all western powers, they now offer you the chance to recruit their units if you have built a port dedicated to trade with them.\n\n\By accepting, you may now wield unprecedented power in Japan. True
dilemmas_localised_second_choice_label_d_bos_treaty_units_all We cannot accept. Convey our regrets to the foreign ambassadors. True
db\dilemmas_tables
Code:
"True" "d_bos_treaty_units_all" "Having\ had\ long\ relation\ with\ all\ western\ powers,\ they\ now\ offer\ you\ the\ chance\ to\ recruit\ their\ units\ if\ you\ have\ built\ a\ port\ dedicated\ to\ trade\ with\ them\.\\n\\n\\By\ accepting,\ you\ may\ now\ wield\ unprecedented\ power\ in\ Japan\." "Such\ a\ military\ understanding\ will\ be\ a\ great\ asset\ to\ our\ cause\.\ We\ accept\." "We\ cannot\ accept\.\ Convey\ our\ regrets\ to\ the\ foreign\ ambassadors\." "Western\ Military\ Missions" "bos_message_diplomacy_positive_active\.tga" "bos_westerner\.TGA" "Accept" "Refuse" "True"
db\cdir_events_dilemma_option_junctions_tables
Code:
"d_bos_treaty_units_all" "30200" "0" "CND_CAMPAIGN" "jap_boshin"
"d_bos_treaty_units_all" "30201" "0" "CND_FIRST_ROUND" "80"
"d_bos_treaty_units_all" "30202" "0" "CND_TECHNOLOGY_RESEARCHED" "boshin_civil_foreign_affairs"
"d_bos_treaty_units_all" "30203" "0" "CND_BUILDING_LEVEL" "bos_port_3_trading_port"
"d_bos_treaty_units_all" "30204" "0" "CND_DILEMMA_NOT_CHOSEN_FIRST" "d_bos_treaty_units_all"
"d_bos_treaty_units_all" "30205" "0" "CND_UNIQUE" ""
"d_bos_treaty_units_all" "30206" "0" "VAR_CHANCE" "7"
"d_bos_treaty_units_all" "30207" "0" "GEN_TARGET_NONE" ""
db\cdir_events_dilemma_payloads_tables
Code:
"FIRST" "d_bos_treaty_units_all" "30208" "0" "p_bos_military_mission_american" "True" "DURATION\[0]"
"FIRST" "d_bos_treaty_units_all" "30209" "0" "p_bos_military_mission_british" "True" "DURATION\[0]"
"FIRST" "d_bos_treaty_units_all" "30210" "0" "p_bos_military_mission_french" "True" "DURATION\[0]"
"FIRST" "d_bos_treaty_units_all" "30211" "0" "BUILDING_LEVEL_EVENT_RESTRICTION" "True" "BUILDING_LEVEL\[bos_port_4_french_trading_district];ENABLE"
"FIRST" "d_bos_treaty_units_all" "30212" "0" "UNIT_EVENT_RESTRICTION" "True" "UNIT\[Boshin_Ironclad_USS_Roanoke];ENABLE"
"FIRST" "d_bos_treaty_units_all" "30213" "0" "UNIT_EVENT_RESTRICTION" "True" "UNIT\[Boshin_Inf_Elite_United_States_Marines];ENABLE"
"FIRST" "d_bos_treaty_units_all" "30214" "0" "BUILDING_LEVEL_EVENT_RESTRICTION" "True" "BUILDING_LEVEL\[bos_port_4_british_trading_district];ENABLE"
"FIRST" "d_bos_treaty_units_all" "30215" "0" "UNIT_EVENT_RESTRICTION" "True" "UNIT\[Boshin_Ironclad_HMS_Warrior];ENABLE"
"FIRST" "d_bos_treaty_units_all" "30216" "0" "UNIT_EVENT_RESTRICTION" "True" "UNIT\[Boshin_Inf_Elite_Royal_Marines];ENABLE"
"FIRST" "d_bos_treaty_units_all" "30217" "0" "BUILDING_LEVEL_EVENT_RESTRICTION" "True" "BUILDING_LEVEL\[bos_port_4_american_trading_district];ENABLE"
"FIRST" "d_bos_treaty_units_all" "30218" "0" "UNIT_EVENT_RESTRICTION" "True" "UNIT\[Boshin_Ironclad_L'Ocean];ENABLE"
"FIRST" "d_bos_treaty_units_all" "30219" "0" "UNIT_EVENT_RESTRICTION" "True" "UNIT\[Boshin_Inf_Elite_Infanterie_de_marine];ENABLE"
This code is for .tsv files to be imported in to the correct tables with PFM 2.0.5 or newer.
EDIT: Edit the original events and set CND_FIRST_ROUND from 24 to some obscure number like 777 or higher, if one wishes. And feel free to edit the CND_FIRST_ROUND in this code to something else than 80, if you like. This works, tried and tested. 
And, this can also be considered as a tutorial on how to create an event.