#1 F-15 is FC3 level so if you swapped the cockpit and add the F-18 pit you will loose full fidelity,or you could also add the F-18 FM dll ect to overcome this.
Have you made a mod before?
Steps are a few to make this work.
You need a mod structure of entry and main Lua files along with other folders for EDM,textures,theme,imput if you include the F-18 FM you need to add a Bin folder and the F-18 FM.
Entry Lua is where you declare goodies of your Mod while main Lua is where you add all sorts of things related to your aircraft information,I recommend you looking into other mods for you to get an idea how these things are made.