Crosstabs are pretty limited, so I doubt that it will do what you want.
If you need an aggregate field for some calculation you can always roll your own, such as:
sum({tablke.field},{table.groupfield})
and use it in a calculation within a formula.
If you post spoecifics there may be a way, but once a crosstab becomes complex, one generally has to create a manual crosstab.
-k