Hello Michel,
Sorry for the inconvenience you're experiencing. Unfortunately, converting or merging a template design from one framework to another isn't a straightforward process. It typically requires advanced frontend development skills and a solid understanding of the Joomla framework.
My suggestion would be to wait for a Joomla 6 compatible update for the Nectar template. Since Joomla 6 is relatively new, it's generally best to avoid rushing the upgrade until all template and extension updates have reached a stable state.
Best regards