Hi,
so the basic concept is that Blueprints build on top of C++ code. This also means that C++ Does not know anything about blueprints. To work around that you normally have to declare a function in C++ and implement it in Blueprints.
Regarding your questing: I am not sure if you can create a Blueprint from a Subsystem. But what you could do, is make a setter function which will actively set the TMap in your MySubsystem. And this could be everywhere, like an Actor in the level who will do this on begin play.
Best
Dominic