Is this possible? I started a fantasy league with 12 teams but after 2 seasons I'd like to add 4 more teams. I didn't see any option to do that. As I workaround I was thinking to start a new league with 16 teams and import players from my existing league but what about their stats? What about coaches? Is this doable?

In between seasons you should be able to change the format to a 16 team league if you want.

Thank you I found that option. Now my question is: Let's say I add 4 teams. Will the game generate new players for those teams or they will just sign free agents and draft picks? How does that work? Is there enough free agents to sign for all new teams?

You either redraft the whole league or you let them sign free agents and use the draft to fill their rosters.