It will depend on the controller fitted but usually, you can drip feed code thru the RS232 using DNC software. Obviously the controller will need to have this function fitted or turned on in its parameters and you'll need the software. Now getting the software might be another story if it's an old controller.?

The other thing is that if you are wanting to run modern tool paths, esp 3D paths like relief carvings, etc that can have huge file sizes it might not be idle because of controller memory size so drip feeding will be restrictive on processing times and can be unreliable. Often these older machines are perfect for retrofitting with a modern controller because the machines are still good but controllers are from the stone age and restrictive if wanting to use modern Cam software/ toolpaths.