I have a county which I would like to add to my duchy.
1.How can I do that?
2.Do I need to have this county as a de jure part of my duchy?
1.How can I do that?
2.Do I need to have this county as a de jure part of my duchy?
Nope, de Jure duchies are set in stoneSo it is not possible to create duchies from counties i have under my control. For example i have 5 counties and I would like to create 2 duchies. I am not allowed to create them and decide which county will be part of which duchy?
You actually CAN decide which county is ruled by which duchy and assign arbitrarily, but without modding, you can NEVER change which county is de jure part of a duchy. For example, I can grant my son County of Paris, Kiev, of Mecca and then grand him Duchy of Jerusalem. Duchy of Jerusalem would now consist of Paris, Kiev, and Mecca, even if they're continents apart. However, Paris, Kiev and Mecca will NEVER EVER become de jure part of the Duchy of Jerusalem... and your son, as the Duke of Jerusalem, will still want control of the actual de jure counties of Jerusalem (Counties of Jerusalem, Acre, and Hebron) and will dislike whoever owns or controls those counties.So it is not possible to create duchies from counties i have under my control. For example i have 5 counties and I would like to create 2 duchies. I am not allowed to create them and decide which county will be part of which duchy?
Then just revoke it by plot or diplomatic interaction.So what can I do if one of my vassals is holding a county which is de jure county of my duchy? Can I assign this county to my duchy by any chance?
No you can't assign a county to a duchy. That's not a thing. You can plot to take the county from him, or just revoke it