Well you wont want to try this. When i was in the US Army, while in basic training we were made to put on our new leather army boots(by no means as costly as yours i'm sure)
and stand in a bath of water and vinegar to soften the leather to prevent blisters and stuff forming on our too soft previously civilian feet. I dont recommend that for your. You can buy stretch tools, or you could take them in to a cobbler(they still have some around)
I had a pair like that once that i would wear with those ugly knee high tight/nylons which would enable me to zip them, and then use the heat from my body to losen up the leather. It took a few wearings and you dont want to go too long in between wearing so they tighten up again, Although once they are stretched they usually are fine. So mu suggestion is to wear them as much as possible until they mold to your calves.