How to import new fields to existing Opportunities?

When importing opportunities Import requires importing Process and Stage values as well as specifying a parent record for the Opportunities. However, if you source file does not have Process and Stage elements, you’d need to set a default value for them

and in order to keep your ACT! data without modifications you’d need to set „Update existing record and do not overwrite existing data“ in the Merge Settings tab.

This way, your new data will be added to new fields and the existing data will be kept without changes.