You can add subtotals to a table using overlapping value sets. In your value set, add an item for each subtotal in addition to the existing items. For example, if you have a value set with single years of age and you wanted to add subtotals for each 5-year age group, you would insert additional items in the value set for each of the 5-year groups:
Then drag the modified value set onto the table. For additional clarity you can format the stubs corresponding to the subtotals to make them standout. In the table below, the font for the subtotal stubs is set to bold.