Help with Transparency

Hello, I am new to recoloring objects and would like to try something specific. I did find the tutorial on transparency but I am looking to add it only to part of an object. I have some doors that I would like to add transparency to a texture making the doors into ones that have stained glass panels with partial transparency. I noticed anything with glass panes has a second texture but I do not know how I would add a second texture to an object?

I included an image below of the door I am working on for reference. Simply put I want to take these doors and add transparency to the center panels. Is it possible?