What do you mean with "add a mat"?
As Terry has pointed out, There are various ways to make borders.
Just clarifying for ease of understanding, we suggest you open the Custom size button and choose "SHORT SIDE" dimension and put 16 or 17.0 inches in there. (Of course, you can use metric sizes if it is easier for you)
The reason we choose short side is because we know the limits of our short dimension; a 17" roll.
Once Qimage knows that, it calculates the horizontal pixels and how much length it will need to print them all.
OK now borders. Easy borders would be to just type in the width of your border in the boxes marked for borders. Find these at the bottom of the Print Tab screen. That assumes you want a border all the way around and all sides equal.
You want a border top and bottom only.
OK, we use mats.
Open mats, (right click of the image) and select Photo Mats Uneven borders.
Here you can select the color and the thickness of your border as you could in the easy border method, but here you can apply the border to your choice of sides.
Last item would be to explain GROW mats. Terry mentioned it before he could realize you were unfamiliar with mats.
When you apply mats, Qimage doesn't know really what you want yet for a final size.
Do wish/ or do you have enough unused print area to keep the print size and add the mat to that making the total print larger? In that case you do what Terry said; tick GROW.
The other selection is SHRINK. This tells QU that you want the entire print including border to be included in the print size and will shrink the picture.
For a pano print, you likely want as much picture as possible so GROW is the likely choice.
Hope this is helpful.
Fred