In order to import the related items to the inventory record , we can make use the same item import method
Go to Setup > Import/Export > Import Tasks > Import CSV Records. After that select the record type for import as item
then choose the csv file which contain the item internla id and its corressponding related item name or internla id
then map the item internal id in csv file to the internla id item record
to map the related item choose the item Presentation fields
Make the item Presentation type as item
then map the related item internal id or name with the item Presentation item
then save the import and check the job status