I would like to add up the 2 input values from "Sales" and "Marketing" into the new column called "Sales and Marketing".
The reason is because there has been a change in the categories of business and the previously separated "Sales" and "Marketing" are now combined as one single category "Sales and Marketing".
Is there a way to add up the 2 figures whilst still maintaining the "input" functionality of the cell? It would not be a ideal situation if I would need to have the users re-enter the values under the new column?
You want to retain the ability to enter data manually in "Sales and Marketing"? You would need to create a line item to add the two values together and import into that line item. I'd advise against that though, avoid imports where possible.
Instead have an override line item, so "Sales and Marketing" would be something like: IF Sales and Marketing Override? THEN Sales and Marketing Override value ELSE Sales + Marketing
So users enter data in to "Sales and Marketing Override" to update "Sales and Marketing"...