Currently, when signing a predefined app image (jpackage --type app-image --app-image <APP_IMAGE> --mac-sign), the --mac-app-store option is unsupported [1]. It seems to be an oversight, as it is applicable to this use case too.
[1] https://github.com/openjdk/jdk/blob/58b601ac4250a455e3f25f8505ead8c130eba642/src/jdk.jpackage/share/classes/jdk/jpackage/internal/ValidOptions.java#L139
[1] https://github.com/openjdk/jdk/blob/58b601ac4250a455e3f25f8505ead8c130eba642/src/jdk.jpackage/share/classes/jdk/jpackage/internal/ValidOptions.java#L139