Yes, you can do this in Monarch. If you create a summary with a measure, double click on the measure field and look for the "Calculation" tab. You have a whole range of standard functions (sum, average, count etc) but also an option called Other. Click on that, and you can enter any formula you like.
That's OK, but what if you have external values that are already aggregated? You could join them to the table so they appear on every line, but then you don't want to Sum() them again in the Summary. So you might use Sum()-Avg().
Hope this helps,