You can actually do this without any manipulation at all if you take care what to use as the platform.
The engine simulates a two-level-dungeon by allowing you to walk on the "ceiling" tiles of the walls in the A4-part of the tileset, and set their directional passability automatically in a way that you can't move from ceiling to floor unless you have a ladder or some other decoration on the walls as an override to that passability restriction.
That is also the reason why some of the A4-walls in some of the tilesets are repeated in A5 - they will look the same but have different passabilities.
But since that only works with these special tiles, you have a more limited selection on how the platform will look. You'll have to study the tileset in the database to find out which tiles of the entire tileset are ceiling tiles, and then find them in the editor window to use them.
And you need to make sure not to place any passable decoration around the platform, because one of the quirks of the program is that not only ladders but every passable upper layer tile will provide the override needed to enter and leave the ceiling - this is a common mapping mistake where for example you could move down a cliff if a flower is placed next to it, or where the "my actors walk on walls in my houses" errors might come from for others.