It's hardcoded into contacts module right now.
You can edit this in modules/CRM/Contacts/ContactsCommon_0.php lines 1243 and further. You have to define a field mapping between contact and company as it's done there.
However I do not recommend editing core modules, but it may be necessary sometimes. I suggest you to create a patch after editing this file, and create some your custom modifications database with patches, so you could apply them after upgrade.
Regards,
Adam
