Mailing List Import Existing Directory Contact

Hi! I'm a newbie. I have trawled the API documentation and can't find an answer so hoping for some ideas from the community. In the UI you can upload a file with existing contacts using the ContactID. I cannot see this option in the API. I have looked at https://ca1.qualtrics.com/API/v3
/directories/{directoryId}/mailinglists/{mailingListId}/contacts and also https://ca1.qualtrics.com/API/v3
/directories/{directoryId}/mailinglists/{mailingListId}/transactioncontacts but both seem to only allow for the creation of brand new contacts.
I don't want to re-add contacts and then dedupe. I just want a way to move people from the directory into a mailing list using the API. Thanks!
Kelly