Placing an expensive piece of carbonfibre, as a replacement insole, will be a waste of time, as you will still get flex from a boot not designed to use step in crampons
Full Scottish winter boots, designed to take step in crampons, have a steel plate actually built into the sole of the boot.
Ive still got a pair of very old 1991 Scarpa Fitzroys, which has a full steel plate built into the sole, and theres a little bit of flex due to it being a leather boot, but its still like walking with a pair of building bricks tied to your feet.
When i have used them during the summer months, which i know sounds a bit crazy, but the thickness and quality of the leather ensures they stay dry far longer than a normal leather 4season boot.
Your ankles and leg shins certainly know about it, once youve worn the things, but the boots are bullet proof.
Boots designed to take full step in crampons, need to be as stiff as possible, and the only way to achieve that, is to construct the sole with a inbuilt steel tensioner.