They should be exported as number format automatically unless you have at least one row which contains a string. If you still struggle I recommend explicitly formatting your expressions using num() or by setting the formatting in the chart properties.
Hi Paul, I've been down this road a few times, this will work if have your number format in your QlikView table exactly the same as in excel. This is usually set in your regional settings on your windows control panel. If they are identical excel will recognize them as numbers. It's usually the digit grouping that causes a mismatch.