The import of .stp file in FLUKA is possible (with some limitations) through flair.
One way is with the use of ANSYS workbench, where a .stp file can be transformed in a .mcnp file.
Then through flair, you have the possibility to import such .mcnp file and the geometry will be transformed in the FLUKA format for you to visualize it.
As said, some limitations might come in this process. Here some examples:
Not all .mcnp bodies can be used (for example, TZ bodies in MCNP are transformed in TCZ bodies in FLUKA which will make the simulation stop)
The regions will be defined but no material will be assigned to them
No VOID and BLCKHOLE regions will be present
Finally let me add that usually the .stp file, particularly if they come from a engineering office, have a lot of details that are not needed, or even harmful, for a FLUKA simulation. I suggest to reduce these details to the minimum before trying this procedure.