Keynote: table cell align right with padding
Increase the Text Inset option shown in your screenshot to add padding to the cell.

This adds padding to all sides (including the right), which may increase row height. It looks like you've already got some spare height to work with, so it shouldn't affect the height if you limit how much you increment it by.

Alternatively if the additional spacing on the top/bottom is an issue and you have numerical data in your cells, you can add fake spacing to the end of numerical data with a custom number format:


Select your cells/column and in the inspector choose Cell → Data Format → Create Custom Format….
Add spaces to the end of the format string. Add multiple spaces and customise their width if necessary.