It is unclear if an adaptation project can be created or not for an application that has already been deployed with an adaptation project.
From the product manual that can be found here:
When you have previously deployed your Adaptation Project but there are further changes in it, you can redeploy it update it directly using the same tool used for deployment.
SAP Business Application Studio (BAS)
ABAP on-premise system
KBA , CA-UI5-FL-ADP-BAS , SAPUI5 Adaptation Project for SAP BAS , How To
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