Welcome to the Community!
At the moment it’s not possible to import into the link to item column. I’m afraid this isn’t supported by our import feature at present.
That said, we do have automations that will allow you to automatically link one item to another. In particular you may find it useful to link these items when the item is created (i.e. at the point of importing them).
This would just require both boards to have some matching information - for example the company name.
Do you think this could work for you?
I hope this helps! Remember to mark as solved if this answers your question 
Best,
Dani