Firstly, if you just want to buy Mod weapons or recruit Mod units for yourself, you can do it with Illegal Weapons or Illegal Units when buy or recruit in the mission, which will search out all available weapons and units, including your Mod.

Secondly, if you want to make new Mod units or weapons as "standard" or preset component, you need to edit the mission file by yourself, as illustrated below:

0. Basic of mission edit.
You need to find the .pbo file of this mission and unpack it to edit. Essentially, now it becomes a new mission.
(a) Soft such as PBOUnpacker help you unpack PBO, and .sqf can be edited by txt.
(b) Unpacked mission folder can also run in game, and you can change the file when game is running, so you needn't pack it.

1. How to add new "Faction" (units).
Setting faction every time is boring, so you can prepare factions in mission file.
Open Setting/PrepareSoldierVehicle.sqf, search TeamArray, and the note will guide you in detailed.

2. How to add standard weapons.
(a)Open Setting>>PrepareCustomStandardWeapon.sqf, you can see I wrote standard weapon in this file.
(b)[["Saiga12K",150,14]] are Name, Money and An Third Unused Variable respectively.
(c)Make sure grenades written in _throwlist rather than _weaponlist. _SniperRifleList is for SubMission sniper.
(d)if()then{} can make file run faster, the file will only run existing and correct code. That's to say, even you write wrong code or not attach certain mod, the file is OK.

3. How to change Other basic variable, such as default value.
In Init.sqs file, there are many variable, most of them can be changed at basic setting of course:
(a)GAMETIME,GAMEDISTANCE,DisL,DisW,InitMoney,InitScore are default time,space,money and so on.
(b)NumGrpWest,LiNuEaSoY are default numbers of major group and background group, as well as the number of team members.
(c)DefGrpAISkill and so on set default AISkill for groups.
(d)DefMoneySoldier and so on are money from kill, and these are also the reference standard for AI recruit and buy.

Thanks for your supporting. Have Fun!