A bankroll is an amount of money, which reduce the risk of going broke due to variance to some acceptable level like 1%. However for this to be possible, you need to be a winning player. And if you only play freerolls, then you are almost certainly a losing player. The only way to become a winning player is to play in regular raked games and prove, you can beat them over a significant sample. Once you have done that, you can have a bankroll. But until then you can only have a budget or a balance in your poker account.
So the answer to your question is, no you can not build a bankroll playing freerolls. But if you are patient and dedicate enough time, you can earn 100$ (or more) playing freerolls. This however is a waste of time for most people, since there are faster ways to earn 100$. Like basically any job. So freerolls are only for fun and maybe a bit of practice. They are not for earning money, and they are certainly not for "building a bankroll". Everyone who claim otherwise basically dont understand, what it means to play with a bankroll.