Extend a standard fiori app from S/4 HANA Cloud in Business Application Studio, possible?

Hello,

We would like to get a standard fiori app that is only available on S/4H Cloud into BAS, extend it and then deploy it on BTP. The application is a freestyle fiori application.
The app can't be downloaded from an on-premise system, since it is only available on s4h cloud.

How do i get this app into BAS, or in WEBIDE?

Many regards.