Marco,
If I understand correctly what you’re trying to do, you will need to loop through the selectedIndices of the checkbox group and then populate the dropdown in code based on those checked items (selectedIndices) of the checkbox group.
https://www.wix.com/corvid/reference/$w.CheckboxGroup.html#options
https://www.wix.com/corvid/reference/$w.CheckboxGroup.html#selectedIndices
https://www.wix.com/corvid/reference/$w.Dropdown.html#options