Since my personal experimentation with fermentation tends to result in biological hazards and minor explosions, I tends to buy my fermented foods. I recently discovered a brand of store-bought jarred kimchi that tastes exactly right... the only problem is that it's not spicy at all. (It might actually have some spice, but I have a very high heat tolerance.) How can I make my kimchi spicier, hopefully in an authentic fashion? Different varieties of pepper have different characteristics and flavors, and I'm trying to figure out the best technique and the variety/varieties which will give me heat without compromising on the flavor. As an example or what I mean -- earlier today I was using the kimchi as a condiment and I mixed it with some sliced jalapenos. Jalapenos taste a lot like green bell pepper, so that was an unfortunate choice, and it wasn't very spicy at all. 
You can add hot pepper flakes. Some advice on what brand from My Aunt’s Homemade Kimchi with a Vegan Kimchi Option:  We’re also pretty particular about the brand of Korean red pepper powder (or hot pepper flakes), gochugaru that we use. We recommend buying Wang Korean gochugaru.   Of course, gochugaru is only mildly spicy. A spicier substitute would be Gochujang, a traditional chili paste which in terms of spice level can range from mild to very hot. 