The Document Grid allows you to specify formats for columns that contain numbers or dates, or other formattable data types.
To set the format of a column, right-click on it and choose "Number Format...".
The "Choose Format" dialog allows you to specify a custom format string.
If you want to save your formatting choices with your custom report, you can use the "Remember Current Layout..." menu item, which is on the Group/Total button next to the Reports dropdown on the toolbar at the top of the Document Grid.
When a report is exported for external tools, the format used for all numeric columns is always the round-trip format ("R"), which provides the full precision possible for the numeric values. This is the format that is used if you choose "Invariant" as the language when exporting a report at "File > Export > Report".