Can you write extensions / modules for VBA used in Excel? -- It's an honest question. I simply haven't used any Microsoft product in a very long time... If not, it's probably not a good delivery mechanism, even if very popular.
There’s various macro extensions and plugins available including .NET but these options tend to have a long history of horrible security problems because they are general purpose languages. While possible to crash spreadsheets with simple formulas they’re not going to steal your local credentials off disk probably.