When migrating a large volume of contacts, organizations, and address records into NPSP, it is essential to consider the following:
Complexity with the Address Object:
Using the Address object to track addresses can introduce complexity, especially if multiple addresses per contact or organization need to be managed. This complexity arises from the need to ensure all address records are correctly linked and maintained.
Impact on Data Storage:
Migrating all historical address information can significantly impact system data storage. Salesforce has storage limits, and large volumes of data can quickly consume available storage. It is important to plan for data storage needs and consider archiving or summarizing historical data to manage storage usage effectively.
Steps for Migration:
Data Preparation:
Clean and standardize the data before migration to ensure consistency.
Identify and remove duplicates to avoid data redundancy.
Data Mapping:
Map the fields from the legacy system to the corresponding fields in NPSP, including custom fields if necessary.
Data Import Tools:
Use Salesforce Data Loader or the NPSP Data Importer for the migration process.
Import data in batches, starting with Accounts, followed by Contacts, and then address information.
Testing and Validation:
Test the migration in a sandbox environment to identify and resolve any issues.
Validate the imported data to ensure accuracy and completeness.
Reference:
"Data Import Strategies for Nonprofits" from Salesforce Help: Data Import Strategies
"Address Management in NPSP" from Salesforce.org: Address Management

To allow staff to use other applications without leaving Salesforce, implementing Salesforce Canvas is the recommended solution:
Salesforce Canvas:
Salesforce Canvas allows you to embed external applications directly within Salesforce

Explanation
https://trailhead.salesforce.com/en/content/learn/modules/user-adoption-metrics/measure-salesforce-usage