Option to import new/update existing on record imports
This is a very standard feature in almost all CRMs and Marketing Automation solutions.
The options at the beginning of import ALWAYS include:
1. Only import new records
2. Only update existing records
3. Add new records and updated existing records
2. and 3. then have an option to "do not overwrite existing fields" and "do not overwrite existing fields with blank values"
Additionally, you should be able to add a field that's not in the import file, but does exist in HubSpot - maybe like Company Owner - and set it to a default value.
This is standard functionality for imports.