This is just a terrible idea from the get-go. To do it right, you're going to need individual mics on each piece of the drum kit. You're going to need a preamp for each mic. You'll probably want a mixer with a minimum of 8 buses to get each drum sound (or stereo pair for overheads) out to your Eurorack. You'll also want a line to Eurorack level module with multiple inputs and outputs... else a bunch of single modules.
Now you have audio going into your Eurorack. If you want to turn your transients to gates, a logic module for each drum sound would be helpful. Once the audio goes above a certain voltage, you'll generate a gate through the logic module... just tap the "or" output. From there you can apply whatever you'd like. Envelope followers were also mentioned. That will generate CV based on the instantaneous amplitude of the signal.
You'll also need a compliment of envelopes, VCAs, or whatever else you're looking to run through.
Honestly, Eurorack isn't the best solution. You'd be better off with a laptop with an 8-in and 8-out interface and software like Ableton. From there you can process, envelope follow, and whatever the heck you want out of your inputs. But using Eurorack is just going to be a cluster with meager results.
Eurorack ain't for everything.