I don't understand your question, or rather, your intent.
If you want to build INFERNO missions (I did just to see the ships that haven't been story introduced for chapters 2,3 and 4. I just plopped one down from each major group and let them go at it while I watched in awe)... Simply open FRED and make the mission as you would for a normal FS2 mission (all the toys are there).
Now you are asking about a adding MOD ship? TO me that means you are adding an outside ship to inferno (like the GTD Saturn). Simply add the pof to the model folder, add the ship info to the table file, place any needed textures in teh map folder wnd bingo. A new ship added to inferno.
Weapons are similar, pof (if needed) in model folder, texture in map folder, now this is tricky sonme weapons need maps in the effects folder (why is a mystery to me, I dunno and don't need to know! LOL!)...Edit weapons table and add new entry (and cross fingers you did it right (make backups!))
I hope this helps you...