You can indeed, but VBA is of course imperative, and VBA is used in part because it is difficult to read/write and considered a poor practice to have any significant chaining of functions in the spreadsheet. The bigger reason it is used, of course, is because Excel's declarative environment does not accommodate all behaviors and users prefer to implement them imperatively. So this sort of reinforces Excel as not being a functional environment.

Being able to write functions doesn’t make a language functional. The hallmark of functional programming is closures as first-class objects, passed as arguments or returned from other functions.

The nearest Excel equivalent would be constructing a formula string for later evaluation, which is far from the same thing. If you wanted to perform lambda calculus in Excel you’d end up implementing your own functional programming language in VBA. Have fun with that :-)