Start by authorized “can create” with studio in the list view of documents to display the “import” button.

Get the model and the id of the record that you want to link (here an example with helpdesk ticket)

You need to know that a document can be linked to any record in any model, so there are 2 fields that are giving that information : res_model and res_id. Export those fields with the external ID of your document 

You will have this 

Complete with id and model of your record

Then import and you will have the link between those two