Using version 5.1 (5683), to my (limited) knowledge of Numbers, there is not an equivalent of the Excel =SUM(ABOVE)
formula in Numbers.
If you primarily want to use the keyboard, start by adding a keyboard shortcut for the SUM formula in Numbers in System Preferences > Keyboard > Shortcuts > App Shortcuts as shown in the image further below.
I gave it: ^S
Now, in Numbers, you can sum any column above the selected cell by pressing: ^S
If you then need to extend the number of cells beyond the cell that has the SUM formula, simple do the following:
- Select the cell that contains the SUM formula and press the delete key.
- Fill in the values of the empty cells as needed.
- Press: ^S
You now have a new sum for all cells above the selected cell you want the SUM formula in.

The shortcut is: Control-S