Hello I would like to know how to design a part in the assembly with for original part a file stp or step?

Normally there is just a plan and convert the entity is shifted and it is set but in this case I could shift some entity on different plan but impossible to do on the whole of the room with for example a sectional view.
Even making a surface recognition does not work.

Do you have answers? one or more solutions for this problem? explanatory pdf I am a taker