While you can't change the default SharePoint site for your team, you can still add any SharePoint document library as a new tab in a channel.
Click Add a tab
next to the other tab names in a channel, then select SharePoint. Do one of the following:
- Click Relevant sites to pick a site from the list.
- Click Use a SharePoint link to paste a link to a SharePoint site or folder.
Choose the document library you want to add.