no you cant, well atleast in the cessna and boeing 747 you cant, i tried, and with the cessna when you go to the back seats, the back of the front seats are invisible, thus seing right through to the open scenery, and with the 747, everything is invisible when you try walk to the back..unless theres a plane thats made with high quality seats etc in the boeing, you cant wall..cause i dont think microsoft would assume that there is going to be a program made to walk around, so they wouldnt waste their time on making seats..
The limits that you can move around in a Virtual cockpit are dependant on a number of factors.
First is the dimensions of the cockpit itself. If you get one of the excellent Meljet add-ons, they come complete with passenger cabins too and even toilets. By no means the first however. Yannick's Falcon 50 I believe has that distinction.
Second, do you have Active Camera Pro? If not, you are limited by Microsoft's command set which means you can't readily access any such passenger area whther it exists or not.
If you have Active Camera and if you have a plane that has a cabin there for you to explore in the first place, then yes you can.