Me and my both both recently bought the game due to nostalgia and we noticed that in LAN it doesn't appear to be possible to call in reinforcements. Is there a way to call them in or a mod to allow it? Any help is appreciated.

I don't know if this has been answered in another topic for you.

But Multiplayer does not provide for Lua Scripting. So the answer is no, reinforcements are not included in Multiplayer.