A year old post. Probably not going to be useful to respond, but in case.
There are some projects to import CAD drawings to MCNP, but they seem to have gotten lost. The latest version of MCNP supposedly has some CAD ability but I cannot report directly. My last use of MCNP was 6.1. If you Google for it you might get lucky, but I would not bet the farm.
Most likely you will be stuck typing it in by hand. Or possibly writing some relatively simple scripts to produce repeating parts of your system like fuel pins that are copied across a core. Or sometimes you will get lucky and be able to copy-paste some parts of your system.
MCNP comes with a utility called VisEd. You can use this to preview your model. It's frustrating because it has limits on how complicated the model can be. If your model gets complicated you will probably first be limited to the 2-D view, and then a little more complicated and it won't work at all. And giving VisEd a model with a bug in it can easily cause VisEd to crash and leave files behind you have to delete by hand to get it to run again.
If you have MCNP set up on a system with a good X-Windows emulator, you should be able to run the plotter that is built into MCNP. This is only 2-D, but is very helpful. Read up in the manual about that.