From a little reading and laymans understanding, it is much more complex/time-consuming Naki-san
A 'Cockpit' externaly viewable internal 3D model would have to with the correct surfaces allocations for texturing and skinning would have to be created and imported into the model, along with reworkingg the model to accept such, even if it were like an empty 'room'/box with one wall missing - so you could see inside it give a fake perception of depth.
While a slighlty less complex task than creating from scratch a new model, an AI flyable plane needs 20 versions/layers of the external model combined to create one aircraft with damage and internal strutctures and differeing visual distancing models that we love IL2 for.
It'd proberly be easier than doing 3D internal model/cockpit to if possible, to cut the doors off, insert a skinnable vertice/mesh facing over the cavernous hole - like a bulkhead that forms a 'cockpit' wall ingamr with a photo skinned upon it of the loading bay. But that is assuming the original model was constructed in such a way that the doors were made seperate to begin with, and not only part of the fusalage.
At least that is my understandings upon it - others know much much more, and can actually use 3D Max/GMAX.