Just like how its not possible to make a regular cut jeans into baggy jeans. You can try to stretch it, but I don't really recommend it as you might tear or pull some strings.
I am not familiar with the pants or the fabric they are made of but I have had some success using a steam iron and a little tugging to make my pants slightly roomier.
You should cut off the final piece of fabric (the dark grey one close to boots) and replace it with a larger one. I see no other way which does not require tailoring. This way you wouldn't have to remake the whole pants, just replace the dark piece of fabric near the boots. I work on tailoring for my custom figures so I can explain you how to do this if you're interested.