How can I associate websites that already exist to my cyberpanel?

Welcome @aymeric75 Happy you are here

CyberPanel stores its websites under /home in any of the formats below:

/home/mywordpressdomain.com/public_html
/home/mysubdomain.com/public_html

You will need to create websites 2 - Creating Website for all those websites in /var/www and move all web files over using ssh terminal or vnc