To add a subtitle to a table, right-click anywhere on the table and choose Format (Table) or choose Format (Table) from the Edit menu to launch the Format (Table) dialog.

Click to add a check mark next to "Subtitle" then click OK. This will add a new row in the table directly underneath the title. Double click on this new row and type in the text for your subtitle. You can then change the format (font, color, alignment, etc…) of your subtitle by right-clicking on it and choosing Format (Subtitle).
To remove the subtitle row, uncheck the corresponding box in Format (Table).