https://sites.google.com/a/cloud4good.com/salesforce-glossary/home/npsp-user-manual/chapter-4-entering-data/npsp-data-importer-tool

When migrating data from a legacy donor management database, the NPSP Data Importer is the recommended tool for loading donor contact information, donation history, and payment information into Salesforce. Here’s why:

NPSP Data Importer:

The NPSP Data Importer is specifically designed for nonprofits using the Nonprofit Success Pack (NPSP).

It supports importing multiple related records (Contacts, Accounts, Opportunities, Payments) in a single file, making it ideal for comprehensive data migration.

The tool provides mapping capabilities to ensure that fields in the import file are correctly mapped to Salesforce fields.

It includes features to handle common nonprofit data structures such as household accounts, affiliations, and recurring donations.

Using NPSP Data Importer minimizes the risk of data discrepancies and ensures that all related records are correctly created and linked.

Salesforce documentation on NPSP Data Importer: NPSP Data Importer