In order to connect the team league with a ranking you do not own, go to the team league admin panel, select the “1.Edit” step and add the ranking. After adding it simply click the “Save” button at the bottom of the page.
If the ranking is “Open”(Link to ranking types - section "e") you will be able to select levels and continue with your team league.
If the ranking is “With Permission”(Link to faq - what is ranking with permission), the ranking first needs to be approved by the owner and the ranking owner should select the levels(Link to faq- 9. How ranking owners can approve Ranking with permission).