Importing Assets after Build

I'm trying to create a program that can dynamically import Excel files into Unity. However, I cannot seem to find a way to Import anything after the build. Being able to import the files after build is core functionality of the program.

So basically my question is: "is It possible to import files after a build?"

I've looked into several ways but I cant seem to make sense of a lot of whats there. Also, the specifics of my program are very complicated and I would like to avoid explaining it if possible so...

Any help would be appreciated. I'm really new at Unity and Scripting and its really hard to jump into something like this all by myself. Also, I'm using C# to script.