You can use the “split cell” keyboard shortcut (alt-enter) at either the top or the bottom of the cell to create a new cell above or below, respectively. Note that when you create a new cell above the current cell this way, you have to press the up arrow to move the cursor into it, but if you create it below, your cursor starts out in the new cell automatically.
This is in some sense inverse to the fact that the “merge cell” key can be used to delete cells.