The Lazarus project, with the cyberzombie Free Mars terrorist might also relate to this discussion.
That episode and others had references to the "cyber experiments" as something from the past, but I don't recall the exact dates.
There's also the Babylon 5 AI Sparky. They say that when the station first went online they tried out an experimental AI, but it was disabled almost immediately, for obvious reasons.
If an AI competent enough to run B5 is possible, and cyberware sufficient to override a human brain, not to mention to run a human body is possible, then I don't think it's a big step to say that a full machine entity is possible. We never saw a specific example of it on the show, but I think it's not a big step to take, if you need/want one in your campaign.
All that being said, however, the holy grail of technology in Babylon 5 is organic tech. A machine entity would likely end up outdated if it's that sophisticated but stops short of building a custom organic body for itself as the next step in its evolution.