With this MoneyMoney extension you can export the transactions in a format that YNAB recognizes.
- In MoneyMoney app open “Help” Menu and hit “Show database in finder” (https://moneymoney-app.com/extensions/#installation)
- Copy ynab_export.lua in extensions folder
- Open MoneyMoney and select some transactions
- Click on "Account" > "Export Transactions..."
- Select "YNAB CSV File"
- Open YNAB, click on import and select the CSV file
- PROFIT!