I don`t think you need to "convert" table to subforms to table to add a column. Example: a simple table with space on the right side to grow. Inserting a column can be done by selecting a column and right-click Insert-Columns to the right/left (or use little + signs)
A basic table does not grow in a column. You need to check if in the column fields the Layout - Expand to fit for the Width is turned on. That might come when you convert forth and back?!?
If your table reaches the end of the page width you might not be able to add another column due to space restriction.
You have to create space on the right border - reduce field width from right to left until enough space is available