INTERSECT A WALL WITH A SLOPE

I created a solid wall as a DGN file.

In another DGN file I have my corridor. When I link the DGN containing the wall, I can cross sections cut perfectly and I can see the corridor and the wall.

Now I need to launch a slope that intersects with my wall.
For this I use the "Define target aliasing" tool but I can not choose my wall.

How do I have to proceed correctly?

Thanks