The 'Vadermaker' site used to have a section about adding laces to the back of the boots. I think he split them down the back of the calves, added the lace holes (with a special tool; a local cobbler could do this easily enough) and threaded them up with boot lace.
I always thought thatwas a good idea, as it gives a bit of 'play' in the tightness of the boot. This is whaty I have done with the boots on my mannequin.
Alternatively, I'm sure a cobbler would add the zips to the boots for you for just a few quid.
Yeah, it shouldn't cost too much and would definately make it easier to get in and out of. I know I struggled for almost 45 minutes the first time I wore mine and tried to get them off after being on my feet for 7 hours. That sucked.