Optionally, you can migrate existing custom ABAP code for add-on development purposes. This custom code migration process analyzes your existing code for cloud-readiness. See How to Check your Custom ABAP Code for SAP BTP ABAP Environment and Custom Code Analysis.
After adapting the code and making necessary changes, you can migrate the code via abapGit. See How to Bring your ABAP Custom Code to SAP BTP ABAP Environment.