Depends on the boot and how its made. I have stretched a few but they are full grain leather uppers without any kind of Rubber sole gaurds. (that is the rubber that runs along the edge of many brands of boots. Case in point I have a pair of Red Wing Elk trackers I had stretched out a bit fixed my hotspot. Your best bet though is to send them to a qualified Cobbler and let them know what you want done. I would recommend Dave Page. He is a legend in the mountaineering boot industry and hes the authorized repair guy for asolo, REI, Vasque ect, ect You can check out his website here. http://www.davepagecobbler.com/
I had him resole a pair of boots for me and it was awesome work. Well worth the 80$ to have it done. If the boot can be altered, or stretched Dave will know.
Not sure if it will work in your situation but worked for an area for me in one of my boots.. Take a quart ziplock or whatever width or length you need and fill with water. Then put it in the part you want to stretch and fill the voided areas with something hard like a rock or what I used was a crown bag with sand in it. Make sure to pack it tight and then lace the boot up a little tighter than your foot wood be. Then throw the boot in the freezer tell water freezes then expands or makes a ice chunk. Then remove and take everything out. Would suggest double wrapping the water just so you don't have leaks.. Like I said worked for me not sure if it will work for your application.
The thing is, I don't want to stretch the whole boot, I just want to widen out a localized area. What I'm thinking of is something like a boot punch typically used for hockey skates.
Leather boots? Stand in creek/water until leather is soaked. Walk a few miles. Wear them all day until they dry. Treat leather after. Not comfortable but I've had a couple pairs that were tougher to break in that this worked for.
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I recommend the best shoe stretcher for bunion and I immensely benefited by using this device. It is a wonderful thing that I have shoe stretcher to remedy this problem. This is a simple device that can expand the length, width and sometimes height of your shoes. It can be used on most shoe materials including leather and suede.
It could likely be done, but you'd have to be very careful not to overstretch and risk damaging the internal waterproof membrane. The membranes are somewhat protected, but typically not super stretchy.