I really don't recommend stacking batteries in any mod unless it was specifically designed for that.
AltSmoke specifically warns against stacking batteries in the Silver Bullet. They even print out the warning in red lettering on the website.
Having said that, before knowing any better and prior to purchasing my Provari, I experimented with stacking the 3 volt RCR123 batteries in my own Silver Bullet for a couple of weeks. Also tried the 5 volt Nimh batteries (require a dedicated charger). You'll quickly find that you will be changing batteries every couple of hours, and the ability of these batteries to hold a charge will quickly degrade, too.
By the time you purchase enough batteries and dedicated charger to reach your goal, you could have best used the money on a Kick and sleeve, or a variable voltage/variable wattage APV. Be aware that these batteries were not designed to be used in the manner you plan to use them. Using any device for other than they were intended carries an inherent risk.
You will be much better served either Kicking your Silver Bullet or purchasing a dedicated variable voltage APV for higher voltage vaping.