Numbers

1. Select all the cells in the column below the spot you want to add cells, at least down to the lowest row that has data. Make sure you select all of them, the ones you miss might be overwritten. A quick way if you need to select all the way to the bottom of the table is to click on the first one then Cmd-Shift-Down Arrow to select from there to the end of the table. Or use Cmd-Option-Shift-Down Arrow to select to the lowest filled row in the column.
2. Click and hold inside that selection box and it will "lift" out of the table.
3. Drag downward and let go when you have the number of new cells you want.

You can use this method to move those cells anywhere, even create a new table out of them.

Adding cells in a row is the same idea. The key combination for selection uses the right arrow instead of down arrow. Otherwise it is the same

While we are on this subject, one thing really neat in Numbers is how easy it is to move cells, groups of cells, or entire rows and columns around. For instance, if you want to swap columns B and C, click on the column header for one of them to select the column, click and hold and the column will "lift" from the table then drag and drop it to its new location. Moving just one cell is pretty much the same procedure (select cell, click and hold, drag and drop) and you can drop it wherever you want it.