I've never had a problem freezing sliced pepperoni. I usually buy a bunch of pepperoni at a time from our local grocery store's meat counter, and divide the pepperoni into amounts that will cover one pizza. I individually wrap those divided amounts with plastic wrap to help prevent any freezer burn, then put all of them into a ziplock freezer bag. When I am going to make a pepperoni pie, I just take one of the divided bunches out of the freezer bag, and let it thaw out on the counter inside of it's plastic wrap. Since they are sliced, they thaw rather quickly, 1/2 hour should be enough time, or you could always put them in the fridge in the morning, they should be ready after a couple of hours.
I have gone as long as 6 weeks in the freezer and never had a problem with frozen, sliced pepperoni. Just don't handle them too much when they are still frozen, they break easily.