As per checking, the ability to apply journal entries (credits) via csv import is still an enhancement logged under Issue 216783 Setup > Import/Export > Import CSV Records > Customer Payments > Ability to map Credits sublist data. For now, we can only apply credits from journals to invoices via UI (Customer Payment).
One work around we could use is to go to Transactions > Customers > Accept Customer Payments and manually apply invoices and credit memos.
To request or vote for product enhancement, please use SuiteIdeas page (SuiteAnswers ID: 30288).