If you are trying to add an existing OneNote notebook to a team and are not presented with the option, it may be because the team was created from a template that doesn't provide it. When creating the team, selecting Cornell Standard Template as your team type will let you create a tab for an existing OneNote notebook later. There is not a way to change the template once the team is created. 

Team types such as Staff, Professional Learning Community (PLC), and Class come with a built-in OneNote notebook that cannot be removed and prevent you from linking existing OneNote notebooks created outside of the team. 

Read more about copying a OneNote notebook to get around this restriction.