Hi,
You can write a multi-player game in SmallBasic with no extension now - different players just use different keys.
What sort of game is this you are thinking of. The limitations of SmallBasic are mainly about performance, so complex (graphically or computationally) games may be impractically slow.
If you want some-one to write an extension, then you will have to be much more specific about what it should do that cannot currently be done, bearing in mind there will be limits to the complexity (due to SmallBasic performance) that will be practical.If you just want something simple like handling a couple of gamepads, then easy.
If you want something that handles multi-player graphic interaction in some way, then this falls into the category of 'you are past SmallBasic'.
EDIT
Spotted the TCP bit in the title so this is about network gaming, using file based data transfer should be fairly easy, but probably there would need to be a dedicated server somewhere for players to both communicate with. Interesting project for some-one wanting an extension project.
EDIT 2
A little looking into it and TCP communication is not too hard between networked computers - so do you mean over the internet or between PC that are networked - e.g. a home network.