Ben,
All you need to do is set up a custom field with a formula that add the
two other fields (columns). Go to Tools/Customize/Fields and, depending
on what type of data you are using (i.e. cost, duration, number, etc.)
pick a spare field (e.g. Number1 if the columns to be added are integer
or floating point values) and hit the "formula" button on the Customize
Fields windows. Then just set up the formula you want. For example,
Number1 = [Number2] + [Number3].
Note that the formula will apply to all task rows, not just a single
task and you will need to select the appropriate roll up formula for
Summary rows on the Customize Fields window.
Hope this helps.
John