frozen dynamite

I'm not an expert, but after some internet sleuthing, I think you got your facts mixed up. Nitroglycerin has a high freezing point, roughly 57 F or 14 C. Once frozen, it is safe to handle (and actually can be difficult to set off intentionally). When not frozen but kept cold, it is actually easier to use without accidental detonation.

However, when it starts to thaw out... that's a problem. Thawing nitroglycerin is as sensitive as nitroglycerin at high temperatures, and if the rate of thawing is too quick it becomes even more unstable.

Regardless of temperature, when handling old or sensitive dynamite/nitroglycerin, it is shock and friction that will set it off. Moving it is not a good idea, but if necessary, packing it into soft material is best. Think a crate full of feathers, fluffy sawdust (not packed sawdust), or the like. The sweated nitroglycerin will pool in the bottom of the storage crate and is extremely unstable; leaving it behind is a good idea if the characters just want the dynamite. If they are trying to make the area safer (as in a house), freezing the nitroglycerin or mixing it with a stabilizing agent is a very good idea. The mixing process alone can set it off, so don't just dump sawdust into the pool of explosive nitroglycerin. There are a number of good stabilizing agents, including ethanol and acetone, though whether your characters have access to those in that time period, I wouldn't know.

On a side note, handling nitroglycerin (including the sweat from overheated or old dynamite) will give you a painful headache. The chemical compound is used in heart medications and can be absorbed through the skin, though shouldn't be fatal unless extenuating circumstances compound the issue.

The site I saw said 10-30% of the resulting mixture should be a stabilizer, but the exact percentage depends on the stabilizer used and the concentration of nitroglycerin (sweated off nitroglycerin would be pretty pure). I'm not sure what exact amount of ethanol you'd need, nor am I familiar with the composition of booze, unfortunately.

On the plus side, hardly anybody else is going to know any of this, either, so you're unlikely to get readers thinking you're bonkers if you get the percentage off by 10% or something.

As a side note, after mixing the stabilizer in, detonating the nitroglycerin will become very difficult (that is, after all, kind of the reason for stabilizing it). You can't use this as an explosive later on in the story.