Unfortunately, there is no export/import function for transferring data from Brightspace ePortfolio to Portflow. So you have to transfer everything manually.
Exporting files from ePortfolio
You can only transfer artifacts (uploaded files) to Portflow. When you export other items from your ePortfolio, such as presentations and forms, you get an XML file that you can't upload in Portflow.
Export multiple files
You can export multiple files at once from ePortfolio. You can then upload them back to Portflow. You can read how to export files in the manual How do I import/export one or multiple items to/from my ePortfolio?.
Building a portfolio in Portflow
Drieam (the company behind Portflow) has collected a number of guides that can help you rebuild your portfolio in Portflow. Check out those guides here.