From Instapaper

Export your Instapaper data

On the settings (https://www.instapaper.com/user) page, click on "Download .CSV file" in the "Export" section. A CSV file will be downloaded (like instapaper-export.csv).

Import your data into wallabag

Click on Import link in the menu, on Import contents in Instapaper section and then select your CSV file and upload it.

Your data will be imported. Data import can be a demanding process for your server.