Zoho Reports: imported table with "formula" field

Zoho Reports: imported table with "formula" field

I set up an imported table in Zoho Reports, with periodical fetching of data.
I tried to define a "formula" field but was not able to.  Is this disabled?  How can I achieve calculations in a report other than sum-ups?  I wanted to subtract the sum of one column from the sum of another column.... thanks.