Adding a new identity service seems to be fairly straightforward.
I don't know what form Linked In IDs take so I just created a basic file (attached) and saved it in
/library/XenForo/Model/IdentityService
This will need to be updated to ensure correct checking of IDs.
I also created a phrase for use with the service:
please_enter_valid_linkedin_id |
Please enter a valid Linked In ID
Then I created the new service in the ACP as shown in the screenshot.
View attachment 11661
From what I can tell it seems to work.
You should be able to repeat that for any other service you want to add.