Hi there,
To add on to Brett’s reponse here, one alternate option you may wish to consider is using API to import the data.
Importing data over Hubspot’s API will retain the record relationships exactly as you have them in your source data, and will link the records together in Hubspot during the import (regardless of the domain names). You can also import all of your records at once, without the use of files.
We offer this as part of our standard data migrations. You can check out how this will work with a free sample here: https://www.import2.com/hubspot/crm
Best,
Ashley