I'm working on a mod but I don't know how to create a puppet and I wonder if  it's even possible to create a puppet

It currently isn't, however, I will look at adding this in soon.

Just so you are aware how creating puppets works, from a code aspect, is by telling the overlord to make another nation the subject (i.e. The UK will have code telling it that India is a puppet). So if you are creating a puppet, you will want to have the overlord created too.