So I was wondering is there any addons that add a 3d cabin to the default airliners that you can walk around in if not it would be a good idea.

I'm not aware of an addon that would give you a 3d cabin for all the default airliners.  A 3d cabin would have to be modeled for each aircraft.  Even if one were thinking about a generic cabin, said generic cabin would have to be modeled as part of each aircraft.  The only option I know that would satisfy your requirement of simulating walking around a cabin would be a payware aircraft.  

19 hours ago, Skye_is_sus said:

So I was wondering is there any addons that add a 3d cabin to the default airliners that you can walk around in if not it would be a good idea.

Welcome to Avsim!

The 3D interiors of each aircraft is made as a part of its model.  If the developer did not model an interior, there is no way of adding an interior.  The same goes for changing anything other than changing textures.  Many aircraft even have the instrument and switch panels modeled as 3D objects, so there is not changing them either.

Sometimes I add the [CameraDefinition.?] section in the .cfg file of an a/c from my hangar to another a/c to get an interior view; It's just the section of the aircraft.cfg which defines these, the description of the view is normaly described next to the section.

Hope this helps.

Berlopez

As others have said the 3D cabin has to be built into the model. So the only way to do this is to search for freeware or payware planes where the description mentions a 3D cabin. 

Most designers (to save system resources) just 3D model the cockpit, wings, engines & tail section. This is why on many planes if you move out of the cockpit theres nothing there. 

The good news is that FSX has a MASSIVE number of planes out there for it and so you can build up a collection of planes with a 3D cabin.