This may not seem very helpful; but, you should go ahead and experiment with it yourself. For me that's the fun and challenge of developing a game with an engine that does have some limitations.
I wanted to do something like this when I first purchased FPSC back in 2007. I had problems with it. I do know that if you spawn the character using the stock script, it probably won't work as I found out. However, what I did was emulated the enemy jumping from the vent. I placed him in the air must outside of the vent and when he was spawned, he fell to the floor (of course landed on his feet) and was ready to do battle. The problem would be that if the player would happen to be looking up, they would see him materialize in thin air.
If you want him to spawn from within the vent or hole that is not very tall, may I suggest you experiment with him in a crouching position and that his cone of view is very high. Also, after he spawns he should be instructed to follow the player but I don't know if he will in a crouching position. However, you can just make him move forward without actually walking until he pops out of the vent and then make him stand up. This will take some customize scripting and experimenting. As far as the sewer cover, I would guess you just have him position on top of the cover and then spawn when you want him to spawn.
If you don't know how to make an enemy spawn, then that's another question.

You can ask that without starting another thread, however.