Declare member extensions outside the class

[tieskedh]

As an extension-function with multiple receivers:

a post from 2014

Extension functions inside class

No, multiple receivers are not supported at the moment. We have this in our backlog, but it's hard to say when we'll get to it. We are also aware of the issue of limiting the scope of extensions (like String.plus) for DSL purposes. We have some ideas about it, but no final decision.

and from 2017:

Extension Function/Property with two receivers

We’ve seen several usecases where such properties could be needed, but so far we’re trying to avoid adding them to the language. Single-receiver extension properties can be hard enough to explain, and this becomes even less obvious. Note that your scenario can be partially solved with a member extension property defined in a base class that all of your activities extend.

So, it wasn’t by design, but I’m not hopefull…

But, maybe it will be possible as inline class?