You can add columns and rows to a table in ScreenSteps using the web editor or the desktop editor. If you are not familiar with how to add a table content block to your article refer to the Add new content to an article article.
Adding columns and rows
1. Click on table to enter edit mode
2. Select a cell > click on row/column options
After you select a cell (1), you will see options to edit the cell, or add rows and columns to the table. Circled below (2) are the options to add rows or columns.
Are there shortcut keys to quickly add rows and columns?
@Anna: There isn't at this time, but I'll pass that feedback along to our product team for consideration!