If you are using Layout, in the library manager, go to tool -> padstack -> Select from Spreadsheet to get the spreadsheet list open. I don't think you can modify an existing pad so you need to create a new padstack. One of the shape options is Oblong. Set the pad width and height to the slot size. This will create a slot with all the same dimensions. Now you can adjust the other layers to make the pad. I usually use one of the pre-defined shapes as a template so I know what layers need to be defined.
I've never required a slot on my boards but I just made one to see if it can be done. Check the results with your board vendor.