it would be added to your empire acting as what is called "Slave" country.
few things to keep in mind:
- indexes needed for levels are computed as average from all countries in empire, thus adding countries makes leveling up to certain level (5) more difficult
- its much much cheaper to conquer c3 than to buy it for 100 GCs. 100 GCs is around 50T in SC$ while conquering costs fraction. dont have exact numbers on me at the moment, but 1/10 or perhaps even 1/20, including cost to buy weapons and ammo required
- decent option to conquest is to check the Cash Market where there are countries with around 20 mil population readily available for 50 GCs
- despite mentioned disadvantages having empire, or conquering c3s, provides also benefits like being able to execute worker exchanges thus being able to tune up un/employment fast and easy