Hello Gametoast,
I am going to explain you how to create interoir rooms and walkways in xsi. This is my first tutorial and I hope it's the first about it.
Anyway. I didn't know how to do that either. But I found a simple trick. You won't need any more tools than XSI.

1. Create your interoir. I am going to make a walkway. I made a cube and deleted the front and back polygons. After that I textured it with one of the Yavin Wall Textures.

2. Select the object and change the following Scaling values from..: to: 3. Now your object should look like in this picture: 4. Now rotate the blue Z-Axis to 180 degress. 5. You know what will be the next step No? Well then....Now export your model as a .msh file and test it in ZeroEditor and in game.
Note: You can only use this trick for floors, walls and roofs. If you want to add pillars, columns or other items then you must export them seperately without using my trick in this tutorial.

This trick works absolutely fine for me. And it's the only way I know.
But I guess using the hex editor is the other solution.

Thanks for reading.

Easier, simpler solution for steps 2-4: select polygons and go to Modify, Poly Mesh, Invert Polygons.

Oh. That's actually all? I never knew that. I always did something else.
Sir. You made xsi a lot easier for me. Thank you.

... select polygons and go to Modify, Poly Mesh, Invert Polygons.

or
Select polygons > right-click > Invert Polygons

Most of the modeling tools can be accessed from the right-click context menu preventing you from digging through menus.

If you have an exterior already modeled and you wanted to create an interior from it, there are several plugins that can create a shell or solidify, add thickness in a couple of clicks, but i've seen better results from using the MoveNormal plugin on more complex geometry.

Do you have any advice for creating windows/holes that connect the interior and exterior? I don't know of an easy way to do that yet.

I usually start by isolating a selection by picking only the walls that will be affected. I use the knife tool in an orthographic view to draw the outline for the opening, then bridge the internal polygons to connect them. I'll worry about cleaning up unterminated edges afterward.

Thanks a lot Ace!

So I followed the interior tutorial but I don't get it and the steps are confusing can anyone go more in depth?

jojo3450 wrote:the steps are confusing

Which steps are confusing?

thelegend wrote:
1. Create your interoir. I am going to make a walkway.

Hidden/Spoiler:

I made a cube and deleted the front and back polygons. After that I textured it with one of the Yavin Wall Textures.

what does it mean to deleted the front and back polygons how would I do that?

To delete polygons you need to select them and then press the 'delete" key.