How to filter out existing records?

I'm trying to take a bunch of records from NetSuite and import them into Square, however I don't want to import any records that already exist within Square's catalog. The only option in the dropdown list when I check "ignore existing records" is to ignore based on whether or not the record has a certain field populated in NetSuite. This is not feasible for me because I need to do this without creating any custom fields in NetSuite. Does anybody know if there is a way for me to filter by comparing a field from my export (NetSuite saved search) to a field in the import (Square Catalog).

Hello @nickrutsky
This is Khaisar from Celigo Engineering team.
Can we get more details on the flow. What is the import API on Square. Is it "Catalog's" API, which you will using.
As per the details, I suppose you are exporting the Item records from NetSuite and creating individual items (including variants) in Square.
If this is the case, then what is the identifier field using which you want to identify the existing items in Square. Generally other users have their unique SKU added to the field "Item Name / Number" on the item record in NetSuite. Similarly do you have any field which holds that unique identifier of each item. If you have such field, then we can make use of the same field rather adding a new custom field.
Please review this and let us know for any further questions.