Is it possible to have command sets for different languages?
Well, yes, you can put what you want in your command set, and you can write as many command sets as you want.
In fact you can even have a command set include another command set.
Iwant to have one command set with functions found within the Blitz3D application, but I would also like to do so with Raknet's code (the one found in its DLL, since our network developer doesn't want to use its own wrapper)
Is that possible or recommendable?
So actually that's not a command sets for different languages that you want but simply a command set with built'in commands from Blitz3D PLUS commands from Raknet, right?
If so, this is naturally possible. You can put in your command set anything that is accessible at the point where you include the "invoker file"
Thus if you have acces to Rakenet's compmands (in Blitz3D this will mean having the right .decls file), then you can put these commands in your command set and your scripts will have access to them.
However from a design perspective, I'm not sure that directly exposing low level network commands to scripts is the best you can do. Usually (but not always) scripts are best used to write the high level logic of the application, and let low level details to the host application. In this context I wouldn't personnaly let scripts handle message sending over the net, and I would rather give this responsibility to the host program.
Naturally this is only a general advice as the best thing to do heavily depends on the specific coontext of your program.
If you have any further question, I suggest you come to the BriskVM forum.