This one is always a bit tricky, I find. Excel is not a great source, so you may need to play around with this a bit.
First, I like to make sure that Excel column is formatted to text. Once that is done, I use a Text(FIELD) function in the Qlik Scripting to try and force the field into text, and retain leading zeros. If that still doesn't work, let us know and we'll give you a hand.
Also, it is helpful to know what you have already tried, so we don't give you options that already are not working.