Just make an object's z-order be 16 or higher and it would behave as a fake object.
The only exception is the entrance trapdoor. A0;They are not affected by z-order, but CustLemm only handles a maximum of 4 (counting from the lowest z-order going up). A0;So the 5th entrance and beyond will not release any lemmings, and therefore fake. A0;And you can overlap multiple entrances to get the same effect of a single entrance. A0;(For example, overlapping 4 entrances in the same position will yield the exact effect as having just one entrance in that position.)
Keep in mind though that some people don't like fake objects, especially when they are intentionally misleading to someone trying to solve the level.