There're 2 different things here:
1) Implement an additional interface for an existing class using whatever syntax
- not doable in a static language for JVM
2) Allow defining class members outside of the class definition
- doable, use cases similar to partial classes in C#,
will require more work on the proposal side (starting from extension vs member, probably just use some other syntax)
and quite some additional support in tooling,
so it needs strong enough motivation.