Export your Readability data
On the tools (https://www.readability.com/tools/) page, click on "Export your data" in the "Data Export" section. You will received an email to download a json (which does not end with .json in fact).
Import your data into wallabag
Import link in the menu, on
Import contents in Readability
section and then select your json file and upload it.
Your data will be imported. Data import can be a demanding process for your server.