Bulk add Sub items into a long list of items

I have a contact list of schools that I am to work with and have a sub item list set as a master that sits within the item (Contact) and is set up ready to use. If I import the contact list of schools from excel, how would I add the sub items to all of the contacts in one go? Is that possible?. I am talking a list of 300 schools, so adding the sub items to each manually isn’t practical. Is there a straight forward way of adding a list of sub items to a new item created, especially when the list of new items are 300 contacts long?