First Time Editing Existing Mesh Need Some Help/Direction

To make a door with glass in it you'd need to clone a door that has glass. Give the glass mesh in your door the same cut number as the glass mesh in your EA-cloned door. Glass uses a different shader than regular objects--a shader is a mathematical voodoo spell games use to try to mimic how light behaves in the real world--so it needs to be a separate mesh group from the rest of the mesh. One thing about doors you need to understand is that they use weight paint. Doors are animated. Anything that's animated needs to be weight painted. If all you're doing is editing an EA mesh it will already be weight painted. But, if you're making a door from scratch you'll want to study how the EA door is weight painted and do yours the same. (It's best to clone a door that is as close to what you're making as possible). The unanimated portion of the door will be 100% assigned to the transformBone weight, while the animated portion will need _bind_doorSwing_[something]_. The name of the _bind_doorSwing bone (weight, bone, joint, and vertex group are all names for the same thing) will depend on what direction the door moves.

As for taking the vampire pack door and giving it real glass, you'd import the mesh into Blender, select the mesh, press Tab to enter edit mode, select the part you want to be glass (an easy way is to select one face then press ctrl + L to select the rest. It doesn't always work, however), then press P to separate the faces. Then assign the correct cut numbers as I explained above.

A door is made up of several meshes. There will be two flat planes that are used for the wall shadow, two frames that aren't animated (one for each side of the door), one or more that are used for glass (if the door has glass) and at least two that are animated. Doors can be a bit complicated, I'm afraid.

You also will need to adapt the texture so that it has an alpha channel with grey in it. Otherwise, the glass won't be transparent. 

If you are brand new to Blender I'd suggest watching YouTube videos. Don't watch videos made by hobbyists. Watch videos produced by professionals such as Blender Guru or FlippedNormals, to name just a couple. They teach how to make meshes but they won't teach how to get your mesh into TS4. For that, use the tutorials here on this site. They are for older versions of Blender as Blender 2.80+ has an interface that is very different. I would HIGHLY recommend you start out with simple meshes such as a painting (there's one here) or another decor item that isn't animated.