Axis used to have a virtual camera tool that did something like this, and you could then simulate any number of Axis cameras, each with a generated MAC address so they seem like different cameras at the same IP but different ports to Milestone. Not sure if they still support this as I haven’t seen/used it in years.
If you are okay with MJPEG and are looking to make it yourself, you could implement the MIP Driver interface in your middleware. There’s an example mentioned here, where you can send video and metadata into Milestone using the MIP Driver:
If you want to test with many video channels without writing something like this yourself, you can look at the StableFPS driver and tools provided in MIP SDK.