I have a faint memory of being able to use VBA functions to calculate values in Excel, like this (as the cell formula):
=MyCustomFunction(A3)
Yes it can. You simply define a VBA function in a module. See http://www.vertex42.com/ExcelArticles/user-defined-functions.html for a nice introduction with examples.
Here's a simple example:
Option Explicit
Function MyCustomFunction(input)
MyCustomFunction = 42 + input
End Function
A1: 2 A2: =MyCustomFunction(A1)