It's possible, but will be difficult. Showing the different floors isn't the hard part; it's giving the zombies the AI to know to climb stairs when the player's on a different floor.
You can try something like this. Turn on a switch when the player climbs to Level 2 (and turn it off on Level 1). Make the second floor graphics events that appear when that switch is on.
Now, use four pages for the zombie. On the first page, don't check any conditions, and set the zombie to "Approach."
On the second page, check the Condition "2nd Floor Switch == ON", and have him move toward the bottom of the stairs (make sure to block the top, so it doesn't pass there!). You'll need some sort of pathfinding script, unless you can figure out a way to do some complex conditional branches.
Still on this page, set it to "Parallel Process" and use conditional branches to check if the zombie reaches the top of the stairs. (Try this: "$game_map.events[ID].x == X and $game_map.events[ID].y == Y" inside a branch.) If it's met, turn Self-Switch A on.
Make a third page, with the Conditions "Self-Switch A == ON". This is when the zombie is on the 2nd floor, but the player is on the 1st. Set the graphic to a lower opacity (to show that it's on the 2nd floor, when the player's on the first). Make him move toward the stairs. On Parallel Process, use a Conditional Branch again to check when he reaches the bottom of the stairs. When he does, turn off Self-Switch A.
On the final page, check both Conditions "2nd Floor Switch == ON" and "Self-Switch A == ON". This is when both the player and the zombie are on the 2nd floor. Use a simple "Approach" move route.
I haven't tested it, but hopefully it should work. Good luck with it!
